InstrEng Instrumentation Engineering: Control Valves

CONTROL VALVE GENERAL CATAGORIES

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Valves can be broadly categorized based on the type of stem movement – linear or rotary type.</div>
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<strong style="background-color: transparent; border: 0px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Globe Valves:</strong> Globe valves are most commonly used liner stem motion type control valves. The flow control for this type of valves is achieved by motion of a plug as shown in the following figure. The shape and type of the plus also determines the valve flow characteristics.</div>
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 <strong style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Gate valve: </strong><span style="background-color: white; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px;">Gate valves use linear type of stem motion for opening and closing of valve. These valves use discs as closure member, as can be seen in the following figure. The faces of this disc can be either parallel or the disc can be wedge shaped.</span><br />
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<strong style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Butterfly Valves: </strong><span style="background-color: white; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px;">Butterfly valves are known for their compact size and low initial costs, which is primarily due to the small wafer and body size of these valves and the simplicity of this design. This valve belongs to the rotary stem motion type of valves.</span></div>
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<strong style="background-color: white; border: 0px; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; margin: 0px; outline: 0px; padding: 0px; vertical-align: baseline;">Ball Valves: </strong><span style="background-color: white; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px;">These valves use a spherical shaped closure member with a cylindrical bore through the member for passage of flow. This spherical closure member has to be rotated by 90 degrees to bring the valve from fully closed position to fully open position. This type gets the name from the shape of the closure member. If the diameter of the cylindrical bore is same as that of the connecting pipe, the valve is known as full bore valve. If this diameter is less than that of the connecting pipe, the valve is known venturi style valve. These valves rotary stem motion.</span></div>
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CONTROL VALVE GENERAL CATAGORIES

Valves can be broadly categorized based on the type of stem movement – linear or rotary type. Globe Valves: Globe valves are most commo...

CONTROL VALVE CHARACTERISTICS

CONTROL VALVE CHARACTERISTICS

The relationship between the control valve opening (also known as ‘valve travel’) and the flow through valve is known as the flow characte...

Control Valves

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<span style="background-color: white; color: #333333; font-family: arial, arial, helvetica, sans-serif; font-size: 14px; line-height: 20px;">Control valves are elements used in process control loops to adjust process variables such as flow, liquid levels, pressure, temperature etc. Control valves essentially consist of a valve and an actuator, more valve control elements may also be present in certain cases. These valves achieve the desired controlling effect essentially by throttling the flow.</span><br />
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Control Valves

Control valves are elements used in process control loops to adjust process variables such as flow, liquid levels, pressure, temperature et...

Typical P&ID arrangement - Control 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 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...

CONTROL VALVE GENERAL CATAGORIES

CONTROL VALVE CHARACTERISTICS

Control Valves

Typical P&ID arrangement - Control Valves

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