Homepage » Plant Processing »

All the right connections

Connection of field devices for partial stroke testing of emergency shut-down valves
All the right connections

Online testing of emergency shut-down (ESD) valves is gaining ground. Many papers and articles have been published under the heading “Partial stroke testing” discussing the partial stroke test (PST) method and its impact on the residual risk to a safety function. In practice, however, the application of the PST method largely depends on the possibility of integrating the field devices into the control environment. In this case, isolation amplifiers play a major role.

Dr. Thomas Karte, Guido König

A number of plant operators already use the online partial stroke test method to test safety instrumented system (SIS) valves in continuously operated plants. By exercising a valve, which normally remains static in the final position in trouble-free operation, with partial stroking, the valve’s ability to move is verified. This exercise offers the added advantage of preventing the valve from becoming stuck due to increased friction at the packing, for example. IEC 61508 and IEC 61511 provide and require a quantitative approach based mainly on the safety availability or probability of failure on demand (PFD) average and the diagnostic coverage rate.
Automated test procedure
Partial stroke testing (PST) is a method where the ESD valve is moved a small amount from the open position. This stroke distance must be selected appropriately. On the one hand, it is necessary to stroke the valve far enough, from 100 % open to 90 % open, for example, in order to verify the operability of the actuator and the valve. On the other hand, the valve must not be stroked too far, since otherwise it would restrict the medium flow and disturb the process while the plant is running.
In principle, the partial stroke test can be performed manually. For this purpose, the solenoid valve is operated to exhaust the pneumatic actuator and the resulting valve movement is limited using a mechanical device. However, important benefits can be gained by automating the test procedure through the use of a positioner which is attached to the ESD valve. These benefits include:
  • The safety or emergency shut-down function is also available during the partial stroke test of the valve with no danger of missing an emergency demand
  • The results obtained can be recorded automatically – eliminating the risk of the results being misread by operating staff. In addition, they can be quantified (stroking time, dead time, uniform valve stroking). They can moreover be used by modern diagnostic tools that make it possible to accurately analyse the valve status and even identify potential failures long before they occur.
  • A remote control function is available for systems designed accordingly. It allows the test to be initiated from the control room, for example, and the test results transferred from the field device to the asset management system.
Basic configuration
The scope of the testing and monitoring functions depends on the selected instruments. Positioners with on-board diagnostics as well as on-board data logging and storing capabilities also allow partial stroke testing of quick-acting valves. On completion of the test, the test results can be transferred asynchronously to an existing asset management system using the Hart communications protocol, for example. This allows existing plants to be upgraded. If the valve is equipped with both a solenoid valve for shut-down purposes and a positioner for testing purposes, the positioner will be able to monitor and record the valve action when fully closing the valve. This type of instrumentation, however, requires a four-wire system to connect the instruments (Figure 1). If the positioner is also used to shut down the valve should an emergency demand occur, a two-wire system will be sufficient for connection purposes. Although such a system allows testing of the solenoid valve function in the partial stroke test, it does not permit the valve action to be monitored when the valve moves to the fully closed position (Figure 2).
Any PST method solution should integrate as smoothly as possible into the automation system of both old and new plants. The need for additional devices, special wiring and a separate system for measuring and storing process data conflicts with the basic idea of IEC 61511, namely to mainly use safety-proven equipment with which users are familiar and which they ideally know from non-safety-related applications. Positioners offering the desired functionality, such as the Type 3730 from Samson, are already available on the market. The process control system is capable of providing and processing the existing signals. And specially designed isolation amplifiers, which act as an interface between the field level and the control level, provide an ideal solution for the required signal conversion as illustrated in the following.
Signal conversion for communication with field devices
In process plants, the ESD valves are typically equipped with at least one solenoid valve. In normal operation, the solenoid is energised and feeds air to the actuator. In the event of an emergency, the logic solver de-energises the solenoid valve and causes the valve to close quickly. Usually, this minimum amount of equipment is complemented by limit switches, which feed the fully open valve position back to the logic solver in normal operation and report the fail-safe position to the logic solver when an emergency shut-down occurs.
Existing plants can be upgraded easily without the need for any additional wiring, as illustrated in Figure 2. However, the signal needs to be converted from 24 V to 20 mA.
This can be achieved in three ways: The voltage output of the I/O board in the logic solver is replaced by a current output. This kind of I/O module is available on the market and integrated into logic solvers from Hima, for example. In this case, an i/i converter can be used as an isolation amplifier to convert the non-intrinsically safe signal to an intrinsically safe signal. These isolation amplifiers are also commonly available in the marketplace. Some isolation amplifier types are also able to transmit the Hart communications protocol and are certified by a recognised authority as suitable for use in safety-related systems.
If the output of the safety instrumented logic solver is intended to remain a voltage output, the voltage signal can be converted to a current signal using a simple resistor, which converts 24 V into 12 mA, for example. Minor signal deviations caused by the resistor tolerance have no impact, since the positioner of the ESD valve interprets the analogue signal level discretely as being on or off. A customary isolation amplifier can also be used in this configuration. It goes without saying that the type, design and connections of the simple resistor have to meet all safety requirements.
The ideal solution illustrated in Figure 3 is not yet available on the market. In this configuration, the isolation amplifier itself would convert the voltage signal to a current signal. The ideal isolation amplifier would also offer Hart capability, certified safety characteristics and linear transmission of the analogue signal.
If retrofitting an additional wire pair is acceptable or if two additional wires are already available in the trunk cable, an integral approach is the best solution. The valve is equipped with both a positioner and a solenoid valve. The most cost-effective and – even more important – the safest solution is to integrate all components in one housing. Apart from the positioner and solenoid valve, the limit switch and the wiring are also accommodated in this same housing.
Figure 4 showcases the integral solution for a rotary actuator. The associated connection diagram is illustrated in Figure 1. In this configuration, the isolation amplifiers are designed as typical u/u or i/i converters.
Hall 10.2, Booth F16
cpp 471
All Whitepaper

All whitepapers of our industry pages

Current Whitepaper

New filtration technology for highly corrosive media


Industrie.de Infoservice
Vielen Dank für Ihre Bestellung!
Sie erhalten in Kürze eine Bestätigung per E-Mail.
Von Ihnen ausgesucht:
Weitere Informationen gewünscht?
Einfach neue Dokumente auswählen
und zuletzt Adresse eingeben.
Wie funktioniert der Industrie.de Infoservice?
Zur Hilfeseite »
Ihre Adresse:














Die Konradin Verlag Robert Kohlhammer GmbH erhebt, verarbeitet und nutzt die Daten, die der Nutzer bei der Registrierung zum Industrie.de Infoservice freiwillig zur Verfügung stellt, zum Zwecke der Erfüllung dieses Nutzungsverhältnisses. Der Nutzer erhält damit Zugang zu den Dokumenten des Industrie.de Infoservice.
AGB
datenschutz-online@konradin.de