Column
Control valve Q & A
About the author
Dr . Hans D . Baumann is an internationally renowned consultant with extensive experience in the valve industry . Throughout his career , he held managerial positions in Germany and France , and his innovative spirit led to the creation of 10 novel valve types , including the well-known Camflex valve . Dr . Baumann has authored 8 books , including the acclaimed " Valve Primer ," and has been granted 115 US patents . He also founded his own valve company , which he later sold to Emerson , and served as Vice President at Masoneilan and Fisher Controls Companies .
Q : Why do parabolic valve plugs have a tendency to slam into the valve seat ?
A : Yes , there are certain conditions where this happens , but never when the stem diameter is larger than the seat diameter . First , such conditions can be avoided when the valve is installed with flow tending to open the valve . Here the fluid force creates a negative feedback . But then , " flow to open " can create violent eddies from turbulence above the valve plug , which may start resonant stem-plug vibration ( and noise ). With liquids , there also may be cavitation damage at the lower part of the valve stem . " Flow to close " avoids the above problems in exchange for possible plug instability due to positive feedback of fluid forces . Yet , there are conditions where it is unavoidable to use a valve this way . One case may be when the valve handles erosive fluids ( such as Bauxite ) or is involved in a chemical process ( such as Urea let-down ). Closing the valve in a system in case of an emergency might also be a reason .
Q : What creates such instability on flow-to-close plugs ?
A : Positive forces are created when the force created by the inlet pressure exceeds the force created by the downstream pressure . When closing forces increase in the direction of travel , we have undesirable positive " feedback ". Here is an example explaining how this happens . ( Please refer to Figure 1 .) Here is a 2 in globe valve , having a parabolic plug . It is assumed that the P1 inlet pressure is 100 psig . The flow is choked , making P2 = 0.5 P1 . Under these conditions , the basic down force ( BF ) is ( 2 2 × 3.14 / 4 – 1 2 × 3.14 / 4 ) x 100 = 235 lbs . In this case , there are 3 travel positions : 0.3 ", 0.6 " and 1.2 ". Next , one has to know the vena contracta area ( avc ) which is affected by P2 . ( consult Figure 1 ). For 1.2 ” travel , they are : 3.14 in 2 , for 0.6 ” travel avc = 1.16 in 2 , and at 0.3 ” travel , avc = 0.45 in 2 . This allows calculation of the net downstream forces ( NF ). Starting with 1.2 " travel , NF = BF – avc × P2 = 235 – 3.14 x 50 = 78.3 lbs . The NF for 0.6 " travel again is BF – avc × 50 , or 235 – 1.16 × 50 = 177 lbs . Finally , for 0.3 " inch travel NF = BF – avc × 50 , or , 235 – 0.48 × 50 = 211 lbs . From the above , one can expect the most instability between travels 1.2 " and 0.6 ". Here
Schematic view of a single seated parabolic plug and seat when traveling from 1.2 . to 0.3 inch .
the rate of change is ( 177 – 78 ) / 0.6 = 168 lb / inch . One way to reduce this danger is to employ an actuator spring with a spring rate in excess of 168 lbs / inch . Note , this only works if the actuator is in the " air to close " mode , ( here the rate of spring force increase exceeds the rate of fluid induced force increases ). NF for 0 travel = 235 – 0 psi × 3.14 = 235 lb . Calculating the rate of force change near zero travel = ( 235 – 212.5 ) / 0.3 = 80 lbs / inch . This shows that the first 0.6 inch of travel are the most susceptible to " slamming ."
Q : Other than changing the actuator spring rate , are there other ways to improve stability ?
A : One other method is : Changing the plug from parabolic to v-port plug , having a more constant vena contracta area around the circumference of the plug .
12 Valve World December 2024 www . valve-world . net