As per EN-746, a Safety Relief 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 Relief 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 Relief valve on the Gas train.
But I have never seen a Safety Relief 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 a 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 Safety relief valve?
Relief valve is a safety device which is mounted on the downstream of the pressure regulator in a Gas train. It releases the small extra pressure that is created due to occasional pressure surges that may occur ahead of Gas regulator. It functions very similar to the Safety Relief valve installed on a domestic Pressure cooker, which releases the excess pressure inside the cooker.
Why should a Gas pressure surge occur (even occasionally) when a Gas regulator is provided? Is it not the function of Gas Pressure regulator to keep the pressure P2 downstream of it constant?
Yes. Technically the function of Gas Pressure regulator is indeed to maintain a constant pressure downstream of it and it keeps doing it most of the time without any trouble.
However, there are some sudden irregularities in the process of Gas Flow which we may term as "transients". For example sometimes there is a sudden increase in the incoming pressure P1 of the pressure regulator. The pressure regulator needs some time to effect the pressure correction at its outlet Pressure P2. Though this correction time is very short (Just a couple of seconds) , there is a momentary increase (jump) in P2 of the regulator which we term as a surge.
Another chance of surge occurs when there are Solenoid valves installed after the Pressure regulators. The closing time of Gas Solenoid valves is always less than 1 second. When the solenoid valves close so fast, there is a "hammer flow effect" due to which a surge occurs in P2 of the Gas regulator. This exerts a heavy backpressure (surge) on the regulator outlet side momentarily. Unless there is a pressure relief valve, this back pressure will directly be exerted on the regulator internal components which are not desirable.
What happens if we don't provide a relief valve?
Pressure surges, though momentary, have a potential of damaging the regulator internal parts like compensation diaphragm and diaphragm plate. Every time there is a pressure surge, it exerts a large back pressure on the regulator creating a high force at the regulator diaphragm. While the diaphragm and other components sustain this back pressure most of the times, the force may sometimes be too large for the components to withstand.
Installation of a Relief valve ensures that the pressure surges are suppressed. Relief valve is a quick acting device which immediately releases the excess pressure in the line, thereby not letting the pressure in the line increase and thus protecting the Regulator from damage.
But if the Gas train already has a Over Pressure Safety Shut Off (slam shut off valve). Why is a safety relief valve needed again?
Slam Shut Off Valve is a Safety shut off device , which closes when the Pressure P2 after the Pressure regulator exceeds certain value. This stops the system completely by cutting off the incoming gas supply. This is OK if the P2 rises for a substantial time, as the rise in this case is not momentary and points to some more serious problem.
Note that opposite to Slam valve, the Safety relief valve does not Stop the Gas flow... it just corrects the Gas pressure.
It is also important to note here that if a Slam Shut Off Valve is provided in a Gas train without a Safety relief valve, there is always a chance that the Slam valve will trip every time there is a pressure surge. This is called a Nuisance shutdown.
What value should the relief valves and Slam Shut off valves be set at?
This depends on the local safety standards which differ from country to country. In some countries the relief valve is set at as low as 1.2 times the regulator outlet pressure P2 while the Slam shut off valve is set at a value 1.4 times.
In India we set the values for Relief and Slam Shut off valve as below:
If Regulator Outlet Pressure = P2; Relief Valve setting = 1.5 x P2 & Slam Valve setting = 2.0 x P2
How to select a relief valve size ?
The flow capacity of relief valve must be 3 to 5 % the Regulator/Gas train flow (whichever is lower) . A Madas DN25 Safety relief valve is sufficient to vent out the excess flow at around 40Nm3/hr of Natural Gas. This makes it suitable to use for Gas Trains with Flow Capacities up to 1000 Nm3/hr.
What happens if I select a relief valve which is larger in size?
It is a common sense that a larger size valve will have a greater flow capacity as a result of which the pressure relief correction will occur in a shorter time. However, the whole action of pressure relief is limited to a fraction of a second as a very small amount of gas actually needs to be vented in order to suppress the pressure surge. Therefore there will be no significant advantage in using a Safety Relief valve of a size higher than required. On the other hand, it will unnecessarily increase your investment in a larger sized valve and associated piping and hardware.
Should one provide an isolation valve for the pressure relief valve?
Very often, users come up with a request for providing an isolation valve for the Safety Relief valve. This helps them to remove the Safety Relief valve from the line in case of maintenance, without having to close down the production process. Providing a Ball valve for isolating a Safety relief valve provides an ease to user in terms of Loss of production down time. If the Safety relief valve malfunctions, the user has an option to isolate it by closing the isolation ball valve, thereby temporarily by-passing the safety relief valve. This relieves the user from the pain of shutting down his system for the sake of one small relief valve.
However, one has to keep in mind that, this is a compromise between Safety and the production downtime. Strictly speaking, a Safety device should never be bypassed by providing isolation or a bypass line. User has to know that till such time the malfunctioning Safety device is repaired or replaced, the Safety of the equipment (and the Operating personnel) is being compromised.
Providing a Safety Relief valve with an isolation valve also risks a chance, that someone may accidentally close and/or leave the isolation valve closed, thereby bypassing the relief valve completely.
The Safety relief valve releases the excess gas to atmosphere…. For Indoor installations is it not unsafe to release the gas just like that inside the plant.
Of course, you cannot release the excess gas (whatever small quantity that the Safety relief valve releases) just like that inside the plant. The outlet of the Relief valve has to be connected to a pipe of same size as the relief valve, and the other end of pipe is expected to be taken out of the factory roof and vented at a safe height.
European standards recommend that all the Vents for the Gas should actually terminate at least 5 meters above the roof level of the plant. Adequate precautions need to be taken to ensure that rain water does not enter these vent lines.
Why are users / customers reluctant to install a Safety Relief valve in Gas Trains?
Apart from the investment in an additional valve (which is actually not so large), the main reason for the reluctance of the Users / Customers is often the associated hassles of installing long pipe lengths to take the outlet of relief out of the plant for venting out the extra gas. Such piping often incurs material and labour cost, which is much higher than the cost of the relief valve itself. In the countries where the Standards of Safety are absent or not stringent, there are instances when the users are not willing to use a Safety relief valve for the above reason. Having done this, they are actually risking a danger to the other components in the gas regulation equipment as also to the Operating personnel.