An interview with Product Manager Mike Northwood explains how operators can implement cost saving vakve partial stroke test systems
Why should operators undertake partial stroke testing?With ESD systems, duplication of control components is very much standard practice; ensuring that should one solenoid valve fail, the mainline valve will still close during an ESD event. It is not typically practical to duplicate the mainline valve and actuator though, meaning that periodic tests need to be made to ensure that the mainline valve will close when required.
Why does Paladon Systems separate the partial stroke test from the ESD function?
This is a contentious question; however, Paladon Systems together with testing houses such as BASEEFA take the view that partial stroke testing is a maintenance function, and not related to ESD. Another major reason is from an operational perspective, let me explain. A partial stroke test is normally carried out between 10 to 15% of the maximum mainline valve stroke. When you consider that some mainline valves need to fully close in 2 to 3 seconds, then 10 to 15% of this stroke does not represent a great deal of time and so raises considerable control issues; especially when trying to manage the momentum of the mainline valve due to its mass and velocity during rapid stroking. Of specific concern to operators is that poor partial stroke control may cause the mainline valve to exceed 10 to 15%, possibly resulting in the shutdown of the entire process
This is a contentious question; however, Paladon Systems together with testing houses such as BASEEFA take the view that partial stroke testing is a maintenance function, and not related to ESD. Another major reason is from an operational perspective, let me explain. A partial stroke test is normally carried out between 10 to 15% of the maximum mainline valve stroke. When you consider that some mainline valves need to fully close in 2 to 3 seconds, then 10 to 15% of this stroke does not represent a great deal of time and so raises considerable control issues; especially when trying to manage the momentum of the mainline valve due to its mass and velocity during rapid stroking. Of specific concern to operators is that poor partial stroke control may cause the mainline valve to exceed 10 to 15%, possibly resulting in the shutdown of the entire process
But what if the operator wishes to test the ESD system?
This is not a problem and can be achieved very easily by pulsing the ESD solenoid valves for a few milliseconds. This will have the effect of creating pressure drops in the pilot lines which are indicative that the solenoid valves are working. A more sophisticated system could use proximity switches directly mounted to the solenoid valves to prove operation.
This is not a problem and can be achieved very easily by pulsing the ESD solenoid valves for a few milliseconds. This will have the effect of creating pressure drops in the pilot lines which are indicative that the solenoid valves are working. A more sophisticated system could use proximity switches directly mounted to the solenoid valves to prove operation.
So what is Paladon Systems’ philosophy for partial stroke testing?
Before I answer that question we should go back to how partial stroke test systems have historically been designed; and what operators could determine from the testing. They have used two mechanical limit switches to detect whether the mainline valve is fully open or fully closed, plus an additional switch to detect 10 to 15% of the total stroke. With this approach you would certainly know when the mainline valve is coming out of its seat and also when it reaches 10 to 15% of its full stroke; however, between these two points there is simply no way to determine exactly what is happening. Paladon Systems’ philosophy is to use a position transmitter or similar device with the ability to give a signal throughout the complete partial stroke test travel and used in conjunction with an approved protocol.
Before I answer that question we should go back to how partial stroke test systems have historically been designed; and what operators could determine from the testing. They have used two mechanical limit switches to detect whether the mainline valve is fully open or fully closed, plus an additional switch to detect 10 to 15% of the total stroke. With this approach you would certainly know when the mainline valve is coming out of its seat and also when it reaches 10 to 15% of its full stroke; however, between these two points there is simply no way to determine exactly what is happening. Paladon Systems’ philosophy is to use a position transmitter or similar device with the ability to give a signal throughout the complete partial stroke test travel and used in conjunction with an approved protocol.
What are the benefits of monitoring the entire partial stroke test?
When the complete valve and actuator assembly is first installed and known to be operating optimally when subjected to the process conditions, a partial stroke test can be initiated and the various metrics such as the time and actuator supply pressure recorded to create a signature of a correctly operating system. The metrics of subsequent partial stroke tests can then be compared against the original signature, and any variances analysed to see if they are indicative of future problems. By doing this, operators have the ability to instigate cost effective preventative maintenance programs, as opposed to the traditional approach of simply servicing valve actuator systems on a regular basis; irrespective of whether the systems actually need it.
When the complete valve and actuator assembly is first installed and known to be operating optimally when subjected to the process conditions, a partial stroke test can be initiated and the various metrics such as the time and actuator supply pressure recorded to create a signature of a correctly operating system. The metrics of subsequent partial stroke tests can then be compared against the original signature, and any variances analysed to see if they are indicative of future problems. By doing this, operators have the ability to instigate cost effective preventative maintenance programs, as opposed to the traditional approach of simply servicing valve actuator systems on a regular basis; irrespective of whether the systems actually need it.
What other aspects should operators consider when selecting a partial stroke test system?
This is another important question, and certainly the partial stroke test information must be able to be recorded so that it can be analysed and acted upon accordingly. To do this, you will need a smart electronic positioner such as Paladon Systems’ PST Controller, and one which has features such as:
This is another important question, and certainly the partial stroke test information must be able to be recorded so that it can be analysed and acted upon accordingly. To do this, you will need a smart electronic positioner such as Paladon Systems’ PST Controller, and one which has features such as:
- The capability to store the information on a removable memory card, or store it internally with the facility to download it via a wired or infrared connection
- Allow initiation of the partial stroke testing by multiple methods such as a handheld infrared controller, digital signal, pre-programed schedule or an external data link
- The ability to undertake data logging during partial stroke testing to ensure full system availability and legislative compliance
- Support for HART, Foundation Fieldbus, Profibus and other types of common communication protocols
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The inclusion of a safety function to ensure that any emergency command will override the partial stroke testing
In closing, could you summarize the benefits to the operator?
There are many; however, in my opinion the major ones are:
There are many; however, in my opinion the major ones are:
- Increased safety and system availability
- Reduced testing costs
- Reduced maintenance costs
- Automatic regulatory compliance