As per EN-746, a manually operated Over Pressure Safety Shut off valve (Slam Shut off valve) is mandatory in a GAS TRAIN if the Maximum operating pressure (Pmax) of any of the components after the regulator is less than the Maximum Inlet pressure to the Gas train. This regulation makes it compulsory for European users to use a Safety Shut Off valve wherever such a situation prevails.
In many countries, there are no defined norms for Gas Safety. In absence of such norms, people tend to cut corners by not installing a Slam Shut off valve on the Gas train.
But I have never seen a Slam Shut off valve on many gas trains which are installed in Europe or the USA?
True. But this is mainly because in most of the European nations or the USA, the maximum inlet pressure allowable inside the plant usually would not exceed 360mBar or 5 psi (as in the USA). Therefore the Inlet pressure to the gas train will never exceed 360mBar. Now since all the downstream components like Solenoid valves, pressure switches or Multiblock valves have a Pmax higher than 360mBar or 5 psi, providing the Slam Shut off valve or Safety relief valve is not necessary.
In India, however, the minimum Gas pressure to the inlet of the gas train is often in the range of 1 to 2 Bar. Downstream Components like Solenoids and Multiblocks have a Pmax of either 360mBar or 500mBar, while Pressure switches on the downstream are rated to Pmax of around 500mBar. This makes it actually necessary to provide both - Slam Shut off valve and Safety relief valve in the gas train.
WHAT IS A SLAM SHUT OFF VALVE?
Slam Shut Off Valve is a device provided at the inlet side of the Gas train before the Gas Pressure Regulator. It is called as “Over Pressure Shut Off (OPSO) valve” or “Safety Shut Off valve” by some. It is installed as a safety measure in case of abnormal operation or failure of the Pressure Regulator.
WHY TO USE A SLAM SHUT OFF VALVE?
As we all know, every gas pressure regulator is set to deliver the gas at its outlet at a constant pressure P2. If for some reason, the Gas Pressure Regulator fails, and then this outlet pressure P2 will rise dangerously high, most of the times at a value equal to the Inlet Pressure P1. This is a potentially dangerous situation, as the downstream components like Solenoid valves and pressure switches are all rated to operate at a much lower pressure. Therefore, there has to be a safety device at the inlet side which stops the incoming gas immediately should a situation as above arise.
SLAM SHUT OFF VALVE WORKING PRINCIPLE
Slam Shut off valve shuts down every time the outlet pressure P2 of a regulator crosses a critical values above its desired value. With this, the incoming gas supply itself stops thereby averting any accident that would otherwise occur. Once Shut down, the Slam Shut off valve needs to be reopened manually to start the gas supply, by operating the manual RESET lever provided on it. If the P2 continues to remain high, the Slam valve will shut off again or the user is not able to RESET it at all.
To put it more simply - Slam shut off valve is a device similar to an Electrical MCB. An electrical MCB trips every time there is an excess current drawn by the load connected to it and stops the incoming Power supply. You have to reset the MCB in order to start the power supply again. If the current drawn by the connected devices continues to be higher than the desired value, the MCB will trip again; till such time the current value does not come below the desired value.
But don’t we already have High Pressure switches in the Gas train which cuts off the Solenoid valves in case of High pressure? Why do we need a Slam Shut off valve again?
Yes. High Pressure switches act more or less in a similar fashion as the Slam Shut off valve. In case of High pressure at the regulator outlet, they cut off the power supply to the Solenoid valves (also installed at the regulator outlet) and stop the Gas flow from going to the Burner. However, there are many reasons why a High Pressure Switch cannot replace a Manual Reset Valve.
But a Slam Shut off valve is an absolute nuisance! It trips and shuts down the Gas supply so often that I lose my expensive production hours. It is also extremely difficult to reset the Slam Shut off valve.
If a Slam shut off valve is tripping repeatedly, it is actually doing its work perfectly. A properly set slam shut off valve trips only when the downstream pressure P2 of the regulator is continuously higher than twice of its desirable value. So the problem could either be a high downstream pressure P2 (user needs to find the reason for the same) or the Slam valve is not set correctly. Once set correctly, Slam valve works perfectly without any problems.
There is a simple technique to reset a slam shut off valve which can be learnt very simply. With a very small effort, you can pull and reset the slam shut off valve with just 2 fingers.
What happens if we do not provide a Slam Shut off valve when required?
What happens if you don’t wear a Safety Belt while driving? What happens if you continue to ride a bike without a helmet? You may still continue to drive through the streets without trouble…. But what if you meet with an accident?
Your equipment may continue to run without a Slam Shut off valve for months together; albeit with a very high risk of an accident and its associated hazard should such a situation arise.