I’ve discussed partial stroke testing (PST) as a diagnostic for the health of a valve in many posts over the years. These posts describe flow applications found in most process manufacturing facilities. But what about high-pressure and hydraulic applications including subsea wells and natural gas pipelines?
I came across an internal article written by Emerson’s Shailesh Sharma, a member of the Valve Automation team. The article highlighted a hydraulic partial stroke and diagnostic system (HPSD) used on emergency shutdown (ESD) valves for a Middle East gas producer.
On the importance of diagnostics for the ESD valve, Shailesh wrote:
The operating integrity of the SIS usually depends on the final control element (an isolation valve). These valves typically remain in one position until needed. In most cases they will never be used to provide emergency isolation which means they can become stuck due to component failure, corrosion or erosion. If the isolation valve does not operate when needed, the process may not be able to stop which presents a catastrophic environment for equipment and personnel.
As we mentioned last week, full stroke testing must be done at certain intervals to uncover issues that partial stroke testing cannot. Increasing this time interval helps to defer some of issues Shailesh highlights, including:
- The process must be shut down or the isolation valve must be bypassed to allow for a full stroke test which is time consuming and costly.
- Mechanical partial stroke testing requires the installation of a travel limiting device and if the device is not removed the isolation valve will not be able to fully stroke when needed.
- ESD valves must be taken off-line to full stroke test which prevents them from performing ESD functions during the test.
- Human intervention is required to perform mechanical stroke tests putting key personnel at risk should an emergency arise.
- Mechanical partial stroke tests are time consuming and costly while preventing resources from being used elsewhere.
For this project, the Valve Automation team provided a hydraulic partial stroke test and diagnostics system. It provides:
…data collection of system pressure and valve position during a PST. An automated PST partially close strokes an open valve and collects pressure and position data to create a torque signature. The torque signature is compared to a baseline torque signature collected at initial system start up.
These periodic tests can help spot potential problems with the valve, the valve actuator, and the hydraulic supply line. Shailesh cited some potential problems:
… higher torque signature can mean that the valve bearings are seizing, the mechanical components of the actuator or solenoid are binding or that there is a problem with the hydraulic pressure system. Knowing about these potential problems allows maintenance crews to fix them BEFORE they occur.
In this installation, the operators and maintenance technicians initiated the PSTs through the DCS connected via EDDL and from pushbutton stations by the ESD valves. They can also be initiated through HART handheld devices, asset management software, and through safety instrumented system logic solvers that support HART communications.
Shailesh closed by noting that if HPSD is used as a hydraulic position in a valve modulating application, then the diagnostics available would include: valve signatures, stem friction degradation, spring rate degradation, seat load degradation, torque degradation, pressure and actuator torque output limits, valve travel limits, static and differential hydraulic pressure, dynamic error band, drive signal, digital valve controller output signal, and valve step response.
Being able to diagnose these conditions continuously and remotely help reduce the chances of unplanned shutdowns caused by the valve, valve actuator, or high-pressure/hydraulic system.