very simple built-up back pressure question and isolation 2

[jrwchem]

Hello,

I am very new to safety valve sizing. I had 2 questions i hope someone could help me with:

1) How do you calculate the built-up back pressure required for the sizing of a psv when you dont know the size of the psv? I thought the built-up back-pressure would be a function of valve size.

2) Is there any advice on isolating a psv for maintance while the unit is still in service? I think the short answer is dont do it, but i would like to offer some alternatives as well. I am seeing a second psv in parallel but that might be considered an 'expensive' solution.

Many thanks,

jrw

 

[TD2K]

Built-up back pressure is calculated based on the valve's relieving capacity (not the required capacity but the calculated capacity for the actual area selected).

Essentially, for an atmospheric relief, it's the backpressure at the valve outlet required to flow the valve's capacity to atmosphere through the exit piping to atmosphere.

If you are discharging to a closed system, it's also the pressure required to get the PSV's capacity through the relief system to wherever it's going, flare, etc. The total backpressure is the sum of the built-up backpressure plus any static pressure that may exist in the flare/relief system.

 

[TD2K]

Sparing relief valves is quite common. Either conventional valves on the inlet and outlet are used. Alternatively, 3-way selector valves can be used for the inlet to select one of the two PSVs. Hamer type blinds can be used on the outlet (essentially a swing type spectacle blind) to isolate the outlet from the flare header when the PSV is removed.

 

[EGT01]

jrwchem,

Just as additional info, the general sequence for design of a relief system would be to determine the controlling relief case and required relief area first then size the inlet and outlet piping for the relief device you select to install.

The selection of the relief device and line sizing may turn into an iterative process unless you have a good feel for what type relief device you will need in the beginning. But in general, when sizing your relief device just assume that the inlet and outlet piping that will be installed will fall within the guidelines for that type device. For example, if you plan on installing a conventional type relief valve, assume the builtup backpressure will be no greater than 10% of the set pressure when sizing the relief valve. Then its just a matter of sizing your lines to meet those requirements.

As additional word of caution, the line sizing can also become an iterative process. When sizing the inlet and outlet piping for the relief device, you need to make a concerted effort to visualize where the relief valve will be installed relative to the equipment it is protecting and the destination of its discharge. Ideally, it is best to have a detail piping isometric showing how the lines will be laid out and all the fittings that will be involved. Otherwise, if you don't have a proposed pipe routing, you will have to make a guess as to how long the lines will be and how many fittings that will be installed. Once the final pipe routing has been determined, then check that it is okay. Too often I've seen what gets installed is more than what was allowed for in the initial design and the line sizes are not adequate. Then the only thing you have to worry about is last minute changes during installation which hopefully won't be many.

 

[jrwchem]

Thank you for the replies.. IT is starting to gel in my head, but it is still a little muddy.

So what i understand is this:

The back pressure is NOT specified initially?

I gues the superimposed back pressure could be determined via hydraulic calculations. But the builtup back pressure is not yet determined.

The valve is sized (more mud here.. but another thread perhaps)... then the inlet and outlet piping is selected to ensure that the builtup backpressure for the given type fo valve (conventional or balanced bellows) does not exceed the recommended percentage of the set-pressure..

Then it seems you are in an endless loop. After the new valve is installed, perhaps the superimposed pressure on the other valves may now be violated due to the new psv.. Perhaps they need to be changed... Am i over thinking this, or can this go on forever?

Thanks,

jrw

 

[JimCasey]

You want the shortest and most direct discharge piping possible on a relief valve.

If you anticipate variable or built-up back pressure, the valve should be of the Bellows-Balanced type. The disc is isolated from the downstream presure by a bellows of the same effective area, vented to atmosphere. Back-pressure effects are thus cancelled. WIthout a balanced valve, back-pressure accumulation will make the valve slam prematurely.Then the backpressure decays or the inlet pressure reaccumulates (or both) and the valve cycles again. This is called chatter and it can be destructive.

There are numerical methods of correcting for the flowrate resulting with back-pressure. The Crosby catalog, and I'm sure many others, publish the appropriate equations. Unlike control valve equations, if you passed high-school algebra you should be able to handle the relief-valve sizing equations.