CONVENTIONAL PRESSURE RELIEF DEVICES

Operation of conventional PSVs is based on the force balance.

Spring load (pre set) = Force exerted on the closed disc by the inlet fluid when the system pressure is at set pressure.

 

 

The above figure shows the force diagram of PSV during normal condition.

The operating mechanism of PSV can be classified into Gas / Vapour service & Liquid service.

Operating mechanism of PSVs with Gas / Vapour service is described in the following steps.

Step 1: As the system pressure approaches the set pressure of the valve, the seating force between the disc and nozzle approaches to zero.

 When the vessel pressure closely approaches to the set pressure, fluid will audibly move past the seating surfaces into the huddling chamber.

Step 2: As a result of restriction of flow between the disc holder and adjusting ring, pressure builds up in huddling chamber as shown in the below figure. It is the operation phase during initial opening.

 

 Step 3: Since pressure now acts over a larger area, an additional force is available to overcome the spring force which causes the PSV to open. This is the operation phase of PSV fully open and is shown in the below figure.

 

 By adjusting the adjusting ring, the opening in the annular orifice can be altered, thereby controlling the pressure build up in huddling chamber.

Step 4: PSV closes when the inlet pressure has dropped sufficiently below the set pressure to allow the spring force to overcome process fluid forces.

Operating mechanism of PRVs with Liquid service is described in the following steps.

Liquid service PRVs do not pop in the same manner as vapour service PSVs, since expansive forces produced by the vapour are not present in the liquid flow. Liquid service PRVs depends on reactive forces to achieve the lift.

Step 1: At initial opening, the escaping liquid forms a very thin layer, expanding radially between the seating surfaces as shown in the below figure.

 

 Step 2: Liquid strikes the reaction surface of the disc holder and is deflected downward creating a reactive force to move the disc holder upwards. These forces typically build very slowly during first 2% to 4% over pressure.

Step 3: Increase in flow leads to increase in velocity which further leads to increase in reactive forces. Increase in reactive forces finally causes the PRV to lift.

Typically PRV will suddenly surge to 50% to 100% lift at 2% to 6% over pressure.

Liquid PRV during operation is shown in following figure.

 

 Step 4: After relieving, the decrease in over pressure leads to decrease in reactive forces which further causes the PRV to close.

During operation of a conventional PSV, the disk lift vs pressure acting on the pressure relieving device is shown in the below figure.

 

Balanced Pressure relief valves

There are mainly two purposes for incorporating bellows in pressure relieving devices.

1.       Minimising the back pressure

Balanced PRVs are the pressure relieving devices which incorporates bellows for balancing the valve disc to minimise the effects of back pressure.

A bellow is attached to the disc holder with a pressure area, approximately equal to the seating area of the disc. This minimises the effect of back pressure acting on it.

2.       Acting as a seal to top parts of PSV

As bellow is attached to the disc holder, this isolate the top parts of the PSV like guide, spring, bonnet within the PSV from the relieving fluid.

The bonnet of a balanced PSV must be vented to atmosphere for the bellows to perform properly.

If any leak is observed from bonnet vent then it is an indication that a bellow is failed as it is unable to isolate the top parts of the PSV.

Incorporating a bellow is very important if there is concern that the fluid will cause corrosive damage to other parts. Where as in conventional PSVs without bellows, during operation, the relieving fluid flows between disc holder and guide, thereby building up the bonnet pressure. This adds variable force to the spring force, which inhibits the PSV lift.

Also this is the reason that a bonnet vent shall be always closed for conventional PSVs without bellows to avoid fluid spillage to outside atmosphere.

The bonnet pressure is reduced by installing educator tube on the hole fabricated on the guide as shown in the following figure.

Balanced PSVs are typically installed where total back pressure (Super imposed + built up) does not exceed approximately 50% of the set pressure.

The force diagram of bellow PSVs can be shown below.

From the above diagram, the force balance equation can be derived as follows.

P1 * A1 = Fs

Where P1 = Inlet pressure acting on PSV disc.

                A1 = Area of the disc.

                Fs = spring force.

If bellows are absent, then 

P1 * A1 = Fs + Pb * A1

Where Pb = Super imposed back pressure.

 Figure reference of force diagram (bellows absent case) can be taken from disc lift vs pressure