InstrEng Instrumentation Engineering

Typical P&ID arrangement - Control Valves

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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh5fzighbma3qS3_augMzq18LWEyLI3pkeiA5I7WCU3fHzqWdyRwE0exm4Av0QDrqj2N9JaScvJkA_3vyT0vtQDXEddiQP5y9Hewr8gqY6YAJFuLO-hPeVMYBguQgoQR6bskCFTdyCe49g/s1600/PID-typical-arrangement-for-control-valves.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="135" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh5fzighbma3qS3_augMzq18LWEyLI3pkeiA5I7WCU3fHzqWdyRwE0exm4Av0QDrqj2N9JaScvJkA_3vyT0vtQDXEddiQP5y9Hewr8gqY6YAJFuLO-hPeVMYBguQgoQR6bskCFTdyCe49g/s320/PID-typical-arrangement-for-control-valves.jpg" width="320" /></a></div>
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<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">The sample drawing presented here represents a typical arrangement generally used to represent control valves on P&ID. Depending on the projects legend sheets, control valves may be represented by globe or gate valves. Here a globe valve symbol is used. First of all a proper valve symbol should be selected to represent the control valve as per the project standards.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Generally, the control valve size is smaller than the corresponding line size. This change in diameter should be clearly indicated in the P&ID with reducer and expander.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Block valves should be provided upstream and downstream of the control valves in case of shutdown and maintenance.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">A drain valve is normally provided between the control valve and upstream block valve. If the control valve is of ‘Fail Open’ type, this drain valve is sufficient to drain the piping segment. If the control valve is of ‘Fail Close’ or ‘Fail in Position’ type, then additional drain valve is required between the control valve and downstream block valve as shown in the sample drawing.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Normally, either a bypass or a handwheel is provided for control valves which are under continuous service. If two or more control valves are installed in parallel, bypass or handwheel is not required.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">The choice between providing either a bypass or a handwheel for the control valve is made based on the size of the control valve. For control valves bigger than a certain size, provision of handwheel is preferred. For control valves smaller than certain size, provision of bypass with block valves is preferred. For control valves on certain critical services, a spare control valve may be installed on the bypass of main control valve. This limiting control valve size between handwheel and bypass is specific for a project and may vary from one project to another.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">If the control valve is equipped with a handwheel, then only the drain between control valve and upstream block valve is sufficient for draining by opening the control valve using handwheel.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Normally globe valve is selected as the bypass valve on the control valve as it allows better control with opening.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Additional details such as failure position, tightness class, # rating etc. are also indicated on the P&ID for control valves, as per the project standards.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">All the guidelines given here are very general and may be modified as per specific requirements of any particular project.</li>
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Typical P&ID arrangement - Control Valves

The sample drawing presented here represents a typical arrangement generally used to represent control valves on P&ID. Dependin...

Typical P&ID arrangement - Pressure Safety Valves

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<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">The sample drawing presented here represents a typical arrangement generally used to represent safety valves or relief valves on P&ID. First of all a proper safety valve symbol should be selected to represent the control valve as per the project standards.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">For protecting equipments that are not spared and equipments that cannot be isolated without disrupting the plant / unit a spare safety valve is recommended to be provided as shown in the sample drawing.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Generally, the safety valve inlet / outlet nozzles are smaller than the corresponding line sizes. This change in diameter should be clearly indicated in the P&ID with reducer and expander.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Block valves should be provided upstream and downstream of the safety valves in case of shutdown and maintenance. Normally provision is made to keep these valves locked or sealed open. The spare safety valve is kept locked or sealed closed, as indicated in the sample drawing.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">A vent valve is normally provided between the safety valve and upstream block valve.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Normally, bypass should provided for safety valves for process or start-up requirements. Type, number and size of bypass valves will depend on the project standards.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Depending on the service handled, the discharge from PSV can be either routed to flare system for hydrocarbon service, for closed/open drain systems or to atmosphere at a safe location for non-hazardous service.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">The inlet lines to the safety valves are always sloped toward to protected equipment and the outlet lines from the safety valves are always sloped towards the flare header / the knock out drum or the safe location.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">When a PSV is connected to the flare system, the inlet line piping should be equipped with a spool piece to facilitate dismantling, as indicated in the sample drawing. For PSVs discharging to atmosphere, this is not required.</li>
<li style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">All the guidelines given here are very general and may be modified as per specific requirements of any particular project.</li>
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Typical P&ID arrangement - Pressure Safety Valves

The sample drawing presented here represents a typical arrangement generally used to represent safety valves or relief valves on ...

Safety Valves P&ID Symbols

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Safety Valves P&ID Symbols per ISA-5.1-2009</h2>
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• Generic pressure safety valve.<br />
• Pressure relief valve.<br />
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• Generic vacuum safety valve.<br />
• Vacuum relief valve.<br />
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• Generic pressure - vacuum relief valve.<br />
• Tank pressure - vacuum relief valve.<br />
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• Pressure safety element.<br />
• Pressure rupture disk.<br />
• Pressure relief.<br />
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• Pressure safety element.<br />
• Vacuum rupture disk.<br />
• Vacuum relief.</blockquote>
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<span style="font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;"><br /><br />Read more: <a href="http://www.instreng.com/instrumentation-basics/606-safety-valves-p-id-symbols.html#ixzz2WGsm8Vp4" style="color: #003399; text-decoration: none;">http://www.instreng.com/instrumentation-basics/606-safety-valves-p-id-symbols.html</a></span></div>

Safety Valves P&ID Symbols

Safety Valves P&ID Symbols per ISA-5.1-2009 • Generic pressure safety valve. • Pressure relief valve. • Gene...

Process Flow Diagram (PFD)

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Process Flow Diagram (PFD) is a drawing which essentially captures the process flow for a processing plant. PFD is used to capture the main process equipments, main process stream, process/design conditions in these equipments and the basic process control scheme in a single drawing. Entire process of a plant can be described using a few interconnected PFDs.</div>
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A PFD should normally contain the following information about the plant process.</div>
<ol style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; margin: 0px 0px 0px 25px; outline: 0px; padding: 0px; vertical-align: baseline;">
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><strong style="background-color: transparent; border: 0px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Main process equipments</strong> with reference tag numbers, name. Process operating and design conditions are also usually provided.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><strong style="background-color: transparent; border: 0px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Main process streams</strong> are normally provided with reference stream numbers. It should be noted that streams are different than lines and stream numbers are not related to line numbers in any way. The streams that normally do not have any flow are indicated with an abbreviation NNF (Normally No Flow).</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><strong style="background-color: transparent; border: 0px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Basic process data for each stream </strong>is sometimes given in the PFD against each stream number. This includes data such as operating temperature, pressure, flowrates, compositions etc. for each process stream. Sometimes, this process data is represented for each stream number, in a separate drawing known as heat and mass balance. Design conditions for a stream are not normally indicated in a PFD.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><strong style="background-color: transparent; border: 0px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Important isolation valves</strong> are also indicated in the PFD. Not all manual valves appear in the P&ID, only a few which can improve the understanding of a process from the PFD. Some valves are indicated as normally closed or locked closed depending on requirement.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><strong style="background-color: transparent; border: 0px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Automatic valves </strong>- motor operated valves / emergency shutdown valves /  control valves appear on the PFD without the associated tag numbers. The purpose is better description of process. Associated control elements are also represented very briefly on the PFD.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><strong style="background-color: transparent; border: 0px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Notes </strong>are added wherever required to improve the understanding of the process from PFD.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><strong style="background-color: transparent; border: 0px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Legend </strong>is a list of symbols used on the PFD with brief explanation. This ‘legend’ can appear on each sheet of PFDs or can appear on a single sheet with other sheets referring to a ‘Legend Sheet’.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;"><strong style="background-color: transparent; border: 0px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Interconnections </strong>from one PFD sheet to another are used for process streams and instrument control signals to maintain continuity between different drawings.</li>
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Some items which appear on the P&ID but may not appear on the PFD are – safety valves, detailed instruments, lines, fittings, drains vents and tag numbers for all of them.</div>
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A variation of the process flow diagram is a utility flow diagram (UFD) which captures the essence of utilities required for the process such as, steam, nitrogen, water etc.</div>
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Process Flow Diagram (PFD)

Process Flow Diagram (PFD) is a drawing which essentially captures the process flow for a processing plant. PFD is used to capture the mai...

Piping and Instrumentation Diagram – P&ID

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Piping and Instrumentation Diagram (P&ID) is a drawing elaborating the details of piping and instrumentation of a processing plant, developed at the design stage. P&ID is later used for assistance for construction of the corresponding plant and for operating that plant. P&IDs of a plant are developed by process design engineers and are followed by instrumentation and piping engineers. A P&ID is normally developed from a Process Flow Diagram (PFD) which captures the basic process flow, at the design stage of a plant</div>
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A P&ID should provide following data to piping and instrument engineers, to construction teams and to the operators:</div>
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<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Equipments – tanks, vessels, heat exchangers, pumps, compressors, columns etc. have to be indicated with type, reference tag numbers, basic design data, spares etc.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Lines – reference tag numbers, piping material class, line size, fluid service, insulation type and thickness etc. Sometimes  process data such as line operating pressure, temperature and flowrates is also represented on the P&ID lines.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Some other piping requirements – such as slope, special insulation such as heat tracing, minimum / maximum piping distance requirements along with their values are also shown on P&IDs.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Manually operated piping valves – valve type (ball valve, gate valve, check valve etc.), valve size, Locked closed/open, sealed closed/open, normally closed/open etc.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Piping fittings – Flanges, reducers/expanders, spectacle blinds, spacers, strainers etc. along with their size wherever necessary.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Drains and Vents – are usually indicated using typical symbols along with their size and type (single valve, double valve etc.).</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Automated valves – Shutdown valves (SDV), control valves, blowdown valves (BDV) are indicated with size if it is known. Also fail position (fail open/fail close/fail in position) is indicated for each of these valves.Actuator connection and type is indicated. Reference instrument tag numbers are attached to every automatic valves.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Safety Valves – Pressure relief valve (PRV) or Temperature relief valve (TRV) are indicated with their instrument tag numbers, setpoints, types are indicated by different symbols.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Instruments – Gauges, transmitters, local indicators, DCS indicators, interlocks and other functions have to be shown in detail on the P&ID. Interconnection between these elements has to be indicated by different types of instrument signals (hardwired signal, soft signal, pneumatic or hydraulic signal etc.). The location of the instrument elements (field mounted or DCS) is indicated by difference between symbols.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Notes – are written wherever required to improve clarity for anyone referring to the P&ID. Sometime ‘Hold’ is used to indicate uncertainty about relevant data.</li>
<li style="background-color: transparent; border: 0px; list-style-type: decimal; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Interconnections (OPC) – are shown between lines (piping OPC) and instrument signals (Instrument OPC) present on two different P&ID sheets. Sometime these interconnectors are also assigned with a unique tag number. For easy identification of the connections between two P&IDs. A connector present on two different drawing connection a line or a signal carries the same tag number.</li>
</ol>
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Because of the numerous details involved in P&IDs for each equipment, usually only one main equipment is shown on one P&ID sheet with related instruments and piping.<br />
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Source: http://www.instreng.com/wiki/Piping_and_instrumentation_diagram</div>
</div>

Piping and Instrumentation Diagram – P&ID

Piping and Instrumentation Diagram (P&ID) is a drawing elaborating the details of piping and instrumentation of a processing plant, de...

Gmail, Drive and Google+ Photos will get 30GB of unified storage

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Google apps team announced that Gmail and Drive both storage will be unified.<br />
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<span style="font-size: 16px;"><strong>Hello Google Apps administrator,</strong></span><br />
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Here's important news: Google storage quota is being unified across Gmail and Drive.<br />
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Beginning May 28th, 2013, Google will change to a new, simpler allowance for storage—one that's combined across Gmail and Drive. We're replacing the current, separate storage allowances—25GB for Gmail and 5GB for Drive—with a single 30GB quota that will cover Gmail, Drive and Google+ Photos.<br />
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Should users need more than the 30GB quota, they can purchase additional Drive storage using the <a href="http://www.google.com/appserve/mkt/7LMJkq7whPbnrmebziua98" style="color: #1155cc;" target="_blank">existing ordering points</a>, and the higher allowance will continue to be spread across all of these services.<br />
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This update is expected to roll out to Rapid Release domains beginning May 28th, 2013, and to Scheduled Release domains starting <span class="aBn" data-term="goog_347694141" style="border-bottom-color: rgb(204, 204, 204); border-bottom-style: dashed; border-bottom-width: 1px; position: relative; top: -2px; z-index: 0;" tabindex="0"><span class="aQJ" style="position: relative; top: 2px; z-index: -1;">June 11th, 2013</span></span>.<br />
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<strong>Here's how the change will affect your users:</strong></div>
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Each user will have a total of 30GB of included storage that can be used across Gmail, Drive and Google+ Photos.</div>
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Users who needed extra storage under the old plans may no longer require it. For example, a Gmail user with just 3GB of email stored will be able to store up to 27GB of files in Drive without a need to license additional storage.</div>
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Gmail inboxes are no longer limited to 25GB. Each user can store up to 16TB of email by purchasing additional storage licenses.</div>
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As before, when users approach their storage quota, additional storage should be purchased to continue using Drive and Gmail normally.</div>
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Also as before, files created in Docs, Sheets and Slides, as well as photos in Google+ smaller than 2048px, won't count against storage quota.</div>
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<span style="background-color: white; color: #434343; font-family: arial, sans-serif; font-size: 13px; line-height: 21px;">For more information, please take a look at the </span><a href="https://www.google.com/appserve/mkt/doc/46tjRzbZtkNh0C6XP66xd6" style="background-color: white; color: #1155cc; font-family: arial, sans-serif; font-size: 13px; line-height: 21px;" target="_blank">FAQ</a><span style="background-color: white; color: #434343; font-family: arial, sans-serif; font-size: 13px; line-height: 21px;"> and </span><a href="http://www.google.com/appserve/mkt/3uMsGYDLN6cVninSLsbZ8Z" style="background-color: white; color: #1155cc; font-family: arial, sans-serif; font-size: 13px; line-height: 21px;" target="_blank">Help Center</a><span style="background-color: white; color: #434343; font-family: arial, sans-serif; font-size: 13px; line-height: 21px;">. And feel free to contact Google Apps support if you have any questions or need further assistance (you can find contact information on the Support tab in your Admin console). </span><br />
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<span style="background-color: white; color: #434343; font-family: arial, sans-serif; font-size: 13px; line-height: 21px;">Thank you,</span><br />
<strong style="background-color: white; color: #434343; font-family: arial, sans-serif; font-size: 13px; line-height: 21px;">The Google Apps Team</strong></div>

Gmail, Drive and Google+ Photos will get 30GB of unified storage

Google apps team announced that Gmail and Drive both storage will be unified. Hello Google Apps administrator, Here's important n...

Instrument air compressor control scheme

<span style="background-color: white; font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;">Instrument air compressor type that usually used in offshore application is a screw rotary type. This type is used due to its large flow capacity and relatively low discharge pressure but still more than enough to supply instrument equipment or any other utilities needed. Since the instrument air compressor didn’t constantly supply a certain flow, thus it need some control scheme in order to operate the instrument air compressor efficiently. There are on-off or start/stop control, load/unload control, and also variable speed control.</span><br />
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<span style="background-color: white; font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;"><b>On-Off or Start/Stop Control</b></span><br />
<br style="font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;" />
<span style="background-color: white; font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;">This is the basic instrument air compressor control. This control scheme is just simply turn off the compressor when the pressure in the instrument air network reach certain high level and turn on when the pressure go down below the set pressure. This control scheme is effectively used when the continuous demand of the compressed air is less than 50% of the compressor capacity. This type of control scheme usually also limited to a small compressor size only (below 10Hp). </span><br />
<span style="background-color: white; font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;">The disadvantage of this control scheme is that too frequent start/stop cycle will make a bearing part worn down.</span><br />
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<a class="highslide" href="http://www.instreng.com/attachments/instrumentation-basics/313d1365420689-instrument-air-compressor-control-scheme-off-instrument-air-compressor.jpg" style="color: #417394; outline: none; text-decoration: none;"><img alt="" border="0" src="http://www.instreng.com/attachments/instrumentation-basics/313d1365420689-instrument-air-compressor-control-scheme-off-instrument-air-compressor.jpg" style="border: 0px solid rgb(0, 0, 0); cursor: url(http://www.instreng.com/highslide/graphics/zoomin.cur), pointer !important; max-width: 800px;" /></a></div>
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<span style="background-color: white; font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;"><b>Load/Unload Control</b></span><br />
<br style="font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;" />
<span style="background-color: white; font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;">This load/unload control scheme is utilizing a load/unload valve. When the pressure in the instrument air network reach a high set pressure, the load/unload valve will close but there are still certain small intake volume goes through the flow restriction. At the same time, the discharge of the compressor bleeds out to the atmosphere through a vent valve. In this way, the compressor will operate on atmospheric pressure only and reduce the power consumption. This control scheme is also completed with a timer so that after several minutes the compressor was unloading, the motor with also shutting down by the controller. This is the most use control scheme for a large number of instrument air users in the instrument air network. By applying this control scheme, instead of having a frequent start/stop it will have an unload condition for certain time (can 30 minutes or more). If after certain time it still unloads, then it will surely off the motor.</span><br />
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<a class="highslide" href="http://www.instreng.com/attachments/instrumentation-basics/314d1365420779-instrument-air-compressor-control-scheme-load-unload-instrument-air-compressor.jpg" style="color: #417394; outline: none; text-decoration: none;"><img alt="" border="0" src="http://www.instreng.com/attachments/instrumentation-basics/314d1365420779-instrument-air-compressor-control-scheme-load-unload-instrument-air-compressor.jpg" style="border: 0px solid rgb(0, 0, 0); cursor: url(http://www.instreng.com/highslide/graphics/zoomin.cur), pointer !important; max-width: 800px;" /></a></div>
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<span style="background-color: white; font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;"><b>Variable Speed Control</b></span><br />
<br style="font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;" />
<span style="background-color: white; font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;">This variable speed control scheme is utilizing a variable frequency drive as a motor controller. When the load in the instrument air network is low, then the pressure transmitter will indicate this and send a process variable to the Variable Frequency Drive (VFD). This VFD will adjust the motor speed to adjust the compressor compression speed. Thus the compressor capacity will go down following the low demand on the instrument air network. When the load is high, the pressure in the instrument air network will be down and the pressure transmitter will send this signal to the VFD. Once again the VFD will adjust the motor speed so that the compressor capacity is increasing. This control scheme is the most precise control of the compressor but having an initial high installation cost.</span><br />
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<a class="highslide" href="http://www.instreng.com/attachments/instrumentation-basics/315d1365420859-instrument-air-compressor-control-scheme-vfd-instrument-air-compressor.jpg" style="color: #417394; outline: none; text-decoration: none;"><img alt="" border="0" src="http://www.instreng.com/attachments/instrumentation-basics/315d1365420859-instrument-air-compressor-control-scheme-vfd-instrument-air-compressor.jpg" style="border: 0px solid rgb(0, 0, 0); cursor: url(http://www.instreng.com/highslide/graphics/zoomin.cur), pointer !important; max-width: 800px;" /></a></div>
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<span style="background-color: white; font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;"><b>Which control scheme we must use?</b></span><br />
<br style="font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;" />
<span style="background-color: white; font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;">As a default, for a large Central Processing Platform or a Refinery Plant application, the load/unload control scheme can be selected as preliminary control scheme. The on-off control scheme can be selected for a small use only while the variable speed control scheme can be selected after some cost comparison calculation with the load/unload control scheme.</span><span style="font-family: Tahoma, Calibri, Verdana, Geneva, sans-serif; font-size: 13px; line-height: 15px;"><br /><br />Read more: <a href="http://www.instreng.com/instrumentation-basics/555-instrument-air-compressor-control-scheme.html" style="color: #003399; text-decoration: none;">http://www.instreng.com/instrumentation-basics/555-instrument-air-compressor-control-scheme.html</a></span>

Instrument air compressor control scheme

Instrument air compressor type that usually used in offshore application is a screw rotary type. This type is used due to its large flow cap...