Change needed in pinch-valve standard definitions
By Gerald Liu
Today’s only existing pinch-valve standards (from ISA) capture years of success to save time and cost for instrument engineers and designers who use these standards in projects related to pinch-valve design, engineering, construction, installation, commissioning, and maintenance. Pinch-valve standards can increase efficiency, productivity, and reliability in plant operations and provide safety to the users and their equipment in application of pinch valves. Yet there is some discrepancy between the existing definitions in ANSI/ISA-75.05.01, Control Valve Terminology, and the draft standard ISA-75.10.03, Installed Face-to-face Dimensions for Shell and Tube Flanged Pinch Valves. Here is a history and an explanation of the need for change.
Historically, the rubber/elastomer sleeve (bladder) and lining allowed pinch-valve manufacturers more freedom with elasticity and isolation to design pinch valves and create face-to-face dimensions for their valves than those manufacturers of conventional control valves.
Elasticity is a property of the rubber/elastomer that allows bubble-tight sealing and shut-off, as well as some flexibility in designing the overall valve body length. The term “soft face-to-face dimension” refers to the linear dimension from the far left side of one flange of a pinch valve to the far right side of the other flange before installation of the valve into the piping system, i.e., when the rubber/elastomer sleeve and the elastic lining on the flanges have not been compressed.
Isolation is not a property of the rubber/elastomer, but manufacturers can achieve complete isolation of the process medium from the valve body with the rubber/elastomer sleeve in pinch valves. Certain pinch-valve designs have mesh wires (sensors) built inside the sleeve for leak-detection in the process. When the sleeve starts to break, such sensors monitor early leaks before breaks are complete.
Another reason pinch valves have their own face-to-face dimensions is they handle fluids conventional control valves could not handle, such as corrosives (acids and caustics used in water treatment/chemical plants), slurries (from sewage systems), and abrasive solids (coarse sands in mining).
There was no standard stipulating what rubber/elastomer material to use for specific fluids in the 1950s; only guidelines existed. While some of the properties overlap each other, pinch-valve manufacturers have their own successful, but untold, stories related to solving unique problems. Pinch valves came into being when engineers saw conventional control valves could not handle corrosives, slurries, and abrasive solids. The engineers worked on their own drawing boards, selected their own rubber/elastomers, and built their own valves. Yet not all pinch-valve manufacturing achieved success, and some pinch valves disappeared from the market. This is why there are five columns of face-to-face dimensions in the current draft standard ISA-75.10.02, Installed Face-to-Face Dimensions for Dual Pinch Flanged Clamp or Pinch Valves (Classes 125 and 150), and four columns of face-to-face dimensions in ISA-75.10.03.
Need for re-defining
The objective of having a standard is not the same as providing one fixed set of face-to-face dimensions for all pinch valves. A standard of face-to-face dimensions can have various sets of dimensions, such as ASME/ANSI B16.10, which has 20 different columns of face-to-face dimensions. Furthermore, having a standard does not mean the standard can never be changed.
One view is shell-and-tube pinch valves are on-off valves, without a physical actuator for their operation, and perhaps should not be considered as a control valve under the definition in ANSI/ISA-75.05.01.
Another view, that of the ISA75.10 subcommittee, is control valves and actuators need to be redefined in broader terms. First, on-off control is a type of process control. Second, current literature illustrates how shell-and-tube pinch valves can be used in throttling (modulating) control when equipped properly with transmitters and controllers, just like any control valve. Third, the definition of control valves in ANSI/ISA-75.05.01 is problematic. Under ANSI/ISA-75.05.01, a control valve is defined as having to have a physical actuator attached. This is not practical in industry. Pressure-reducing valves have no actuators, but they are still control valves. Similarly, shell-and-tube pinch valves are operated by a pressure signal; they work without an actuator and can perform modulating controls like a control valve. To broaden the application of control valves, the ISA75.05 subcommittee should revisit the definitions of the terms “control valves” and “actuators” in ANSI/ISA-75.05.01.
Thus, in ISA-75.10.03, control valves and actuators are re-defined as described above. Control valves are any valves operated by an instrument signal. Actuators are transducers for converting instrument signals to displacements. With these definitions in place, ISA75.10 can establish the new ISA-75.10.03.
ABOUT THE AUTHOR
Gerald Liu, P.E. (email@example.com) is the chairman of ISA75.10 and lives in Calgary, Alberta, Canada. If you are interested in joining the efforts of ISA75.10, contact him or Ellen Fussell Policastro (firstname.lastname@example.org.)