A question came into the ISA’s Safety email list concerning the use of valve positioners in partial stroke tests for valves used in safety shutdown applications. The person asking the question wrote:
I was informed that PST using positioner such as the Fisher/Emerson has got problem when first opening the valve because the CV value of the positioner is small so it will take a very long time to open for large valves/actuators. Please advise if this comment is valid.
Partial stroke test (PST) is a diagnostics function, which is performed on line, in service, hence minimum process interruption is highly desired. If the “FAIL OPEN” valve is opened with sudden jerk, it can create a blurp or surge of process, which may create unwanted results–i.e. liquid service, sudden opening or closing of valve, which may lead to the “water hammer” phenomena.
The Emerson – Fisher DVC6000 SIS is specifically designed to stroke the valve during PST using a RAMP algorithm, which stabilizes the process while lifting a valve from its seat. Certainly, concern may occur if a DEMAND arises during a stroke test. The DVC6000 SIS has built-in safeguards to immediately take a valve to its safe state with the desired pneumatic path. In fact, the DVC6000 SIS has a unique feature, which allows engineers to configure stroking speed as desired by a few industries. The Digital Valve Controller is smart enough to differentiate between a Partial Stroke Test and a Safety Demand.
We have seen a few oil producers using DVC6000 SIS with external flow restrictor in pneumatic line to slow down valve travel to have process equilibrium during a SAFETY DEMAND so that slam shut action does not cause piping breakage as well as a loss to equipment by suddenly cutting down fuel. This may possibly lead in some of the process spoilage of catalyst.
Depending upon need of Process Safety Time (PST) of a SIF [safety instrumented function] loop, a few valve applications may require a Process Safety Time of less than 2 seconds. In such cases, it is always recommended to use external devices to allow additional quick release of air volume from the actuator to meet the time line.
Immaterial of Digital Valve Controller manufacturer, positioners always operate during normal condition at full pressure load, which as per characteristics, allows air to bleed. If a positioner of high exhaust and fill capacity is used, it will bleed excessive air. The Emerson – Fisher DVC has been designed in such a manner that it has only an air consumption of 2.1 scfh at 20 psi compare to positioners with a higher Cv, whose bleed rates are exceptionally high (> 20 times of the DVC6000).
Once again, a Partial Stroke Test is diagnostics, which can be considered “Safety Related” but NOT “Safety Critical”, hence, during test valve opening time should not be pose any challenges.
Should you need more technical clarifications, I can provide details on one to one basis.
Another list member noted this issue in his response to Riyaz:
It is a very simply matter to restrict the exhaust rate of the diaphragm during the partial stroke test, and thus slow the stroking speed to whatever speed is desired. This would eliminate any concerns of overstroking, or rapid valve movement. However, since we are in the hysteresis range (15%) of the valve, problems resulting from rapid movement should be minimal to start with.
Using a quick exhaust device to vent the valve diaphragm (to compensate for the small Cv of the positioner) must be accounted for in the PFDavg calculation of the final element for the SIF. This simplex device is a critical part of the SIF and cannot be ignored.
Riyaz addressed this concern [again with my light edits]:
As explained in my previous email, when using microprocessor-based device or any other means to initiate PST for Safety Shut Down valves, in line in service, Quick Exhaust Valves (QEV) typically show instability. Based on lab tests and past field experience, it has been observed that the QEV comes in action for water column Pressure Difference, which may lead to uncontrolled travel, during PST test.
In such situations, it is preferable to use a Volume Booster, which helps in both directions (opening and closing). Also as clarified by [another email responder], that the volume booster is checked during a PST.
As you rightly said, Volume Booster kinds of mechanical devices are simplex. These Type A devices have been in the field over the past four decades, and for sure have already established its all-possible failure modes in its operating run. This would provide beneficial leads to manufacturers to incorporate in design. Therefore, it is less likely that any reliability issues may not have been taken into account and corrected by manufacturers of mechanical components.
Some additional questions came in about the certification reports. Exida’s Dr. Bill Goble pointed the questioner to where the partial stroke test certification reports are located on the Exida website. Here are the links to the DVC6000 assessment report and DVC6000 certificate for use in emergency shutdown (ESD) partial stroke valve monitor applications.
If you’re involved with process safety at your plant, I hope these clarifications by Riyaz help to address similar questions that you may have.