Valves are essential components for the safety and operation of plants. Tests known as FST (Full Stroke Test) and PST (Partial Stroke Test) are cornerstones of programmed maintenance.
There are many techniques, standards and procedures well known to maintainers, instrumentalists and engineers that make it possible to identify critical equipment while an emergency occurs, and to increase their reliability and integrity. When it comes to critical valves inside the installations, like locking and safety valves, the problem is conducting periodic functional tests.
Generally, these valves can take a long time to be operative. They normally suffer a decay both in terms of material integrity and design suitability because they are subject to weather conditions or can be installed in aggressive and corrosive environments.
FST
During scheduled maintenance periods the valves are activated, flow rates are in the full open and closed state, allowing to check the possible block of the stem, the air leakage on the actuator, the watertightness of the closure, the integrity of the valve and actuator and the signal transmitted to the control panel.
These controls, together with other secondary ones, contribute to define the Full Stroke Test (FST) which provides a wide range of parameters to assess valves. They also allow the planning of correct preventive maintenance strategies before an emergency occurs.
Unfortunately, for obvious reasons of productivity and profitability, industrial plants cannot afford many FSTs that take months or even years to be executed. FST tests requires both plant’s process and additional equipment to be stopped, such as manual locking valves, bypass pipes, electro-valves, mechanical stroke control devices, not to forget logistics and the staff involved. Costs are therefore a critical factor and often an important constraint.
PST
The simplest, cheapest and most reliable alternative is the Partial Stroke Test (PST). PST means to partially move the valve axis and measure the actions required to do so. In addition, you can measure the valve response time or check whether the valve is locked or whether the pneumatic actuator is correctly pressurized.
These controls do not necessarily require on-site inspections. There are various methods of PST implementation: field, remote, self-initiated and manual. Some manufacturers offer panels with buttons and lighting systems to test the functions directly in the field.
The test can also be performed from a control room with an application software and a Distributed Control System (DCS) or a system using Hart commands, with intelligent instrumentation and field devices.
The software used for PST testing must be based on functional safety standards (IEC 61508, IEC 61511, ANSI/ISA-84.00.01), better if proposed and validated by the same technology provider.
According to documents published by the OREDA (Offshore Reliability Data) organization, PST can detect 70% of the valve problems that statistically occur with highest frequency. The duration of the test depends on the process and the interference must be minimal. In many cases, 15% of valve opening variations already allow to identify potential problems. The typical range of a PST is 10% to 20% of valve movement.
Considering the growing interest in SIS instrumented safety systems, it can be said that the PST is a consolidated method that influence the calculations related to PFD (Probability of Failure on Demand), whose value is used to determine the SIL (Safety Integrity Level).
In any case, a PST applied to emergency stop valves (ESD) improves the performance of the Safety Instrumented System (SIS). In this context, we speak of Partial Valve Stroke Tests (PVST). The method used by intelligent valve positioners to perform the PST test is called the Dynamic Ramp Method.
The valve moves in response to the user-determined setpoint, while the positioner reads the valve position through the Hall effect position sensor, without mechanical contact. At the same time, the positioner measures the applied pressure required to move the valve stem. After reaching the highest setpoint, the positioner returns to the original position.
PST, advantages and disadvantages
A PST is required to achieve a higher level of safety integrity (typically SIL3), where PFD calculations cannot achieve the desired objectives.
The primary objective of the PST is that the valve actuator moves when required without blocking itself, since it didn’t move for long periods due to the high TAR (Turn Around Time).
The main disadvantage of all PST test systems is the increased chances of accidental activation of the safety system leading to a shutdown. Different techniques can mitigate this problem in different ways, but all systems have an inherent risk.