Column
Control valve Q & A
Q : Our process engineer has asked me to specify a control valve for a new system he is designing . He has given me some process data , including the specific gravity of the process medium , its vapor pressure at the flowing temperature , and the maximum and minimum required flow rates . When I asked him what the pressure drop across the valve would be at the specified flow rates , he said I should select a valve that would create minimum pressure drop and suggested I consider a full port ball valve .
About the author
Jon F . Monsen , PhD , PE , is a control valve technology specialist with more than 45 years of experience in the control valve industry . He has lectured nationally and internationally on the subjects of control valve application and sizing . Jon currently hosts a website , www . Control-Valve- Application-Tools . com where he freely shares articles , training and professional development materials , and Excel worksheets that might be of interest to those who use or specify control valves . He can be reached at cvapptools @ gmail . com .
A : Perhaps the most misunderstood area of control valve sizing is the selection of the pressure drop , ∆P , to use in the sizing calculation . The pressure drop cannot be arbitrarily specified without regard for the actual system into which the valve will be installed . What must be kept in mind is that all of the components of the system except for the control valve ( pipes , fittings , isolation valves , heat exchangers , etc .) are fixed , and at the flow rate required by the system ( to cool a hot chemical to a specified temperature , maintain a specified level in a tank , etc .) the pressure loss in each of these fixed elements is also fixed . Only the control valve is variable , and it is connected to an automatic control system . The control system will adjust the control valve to whatever position is necessary to establish the required flow ( and thus achieve the specified temperature , tank level or whatever ). At this point , the portion of the overall system pressure differential ( the difference between the pressure at the beginning of the system and at the end of the system ) that is not being consumed by the fixed elements must appear across the control valve . The correct procedure for determining the pressure drop across a control valve at the flow rate for which you plan to perform a sizing calculation , is to start upstream of the valve at a point where
Figure 1 . Analysis of control valve inlet and outlet pressure for control valve sizing .
the pressure is known ( for example a pump where the pressure can be determined from the head curve ) and subtract the pressure loss in each of the fixed elements . When you get to the valve inlet , you know P 1
, the pressure immediately upstream of the valve . The next step is to go to a point downstream of the control valve where the pressure is known ( for example , a tank where the head is known ) and then work upstream toward the control valve , adding the pressure loss of each of the fixed elements . ( You are adding the pressure losses because you are working in the direction opposite to the flow ). When you get to the valve outlet , you know P 2
, the pressure immediately downstream of the valve . The actual pressure drop across the control valve is the difference between the upstream and the downstream pressures , that is ∆P = P 1
– P 2
. If you plan to perform sizing calculations at more than one flow rate ( for example , at both maximum and minimum design flows ), you must repeat the calculation of P 1 and P 2 at each flow rate , since the system pressure losses ( and pump head ) are dependent on flow . This procedure is depicted in Figure 1 . If the process engineer is unhappy with the sizing pressure drops you have used in your sizing calculations , he needs to reconsider his choice of pump or pipe size .
∆ ∆
14 www . valve-world . net Valve World August 2023