rupture disk device system 1

[38621]

On a 30" line of 4*10^6 lb/hr liquid water at 350 psi, we intend to instael two 10" rupture disk in paralel. The idea is to protect the line from pumps over pressure.

We consider using isolating valves around the rupture disk for easy replacment after bursting. Is this wise? Is there any regulation agenst it? Could you advise us to instael any other devise like a surge or thermal growth accumulator?

Thanks in advance for you advice.

 

[greg87]

A pressure relief system should be the last level of protection for equipment and personnel against hazards due to release of energy or fluids. You want it to work when all else has failed, including failure of personnel (as in failing to reopen a valve after it has been closed for maintenance) So using isolating valves in relief lines is not a good idea (although it can be done in with the right design and controls). However, a relief system should “operate” only under worst case situations,which hopefully never happen, so you should not be replacing rupture disks with any regularity – if at all (although depending upon pressure cycling etc, they may need to be replaced for PM, which should be infrequent if they were specified correctly.)

If you really think that overpressure could happen with some frequency, my impression is that you should be looking at other ways of limiting the maximum pressure of the pumps, which could be mechanical or instrumentation/control based. Without knowing anything about your pumping system, its hard to say what the options might be. It may very well be that these features/changes would eliminate the need for rupture disks (or relieve valves) altogether – but that depends on the type of features, and the consequences of overpressuring the pipe.

Also, those 10 inch disks seem awfully large. What kind of flow rate could you need to be relieving if you need a ten inch (or two ten inch) to relieve with a relieving pressure of (at least) 350 Psig discharging (I presume) to atmosphere?! How were those 10 inch sizes determined? Further, I don’t understand your reference to the surge device – that would have a completely different application then relieving pump flow.

 

[38621]

Thanks greg87. Doesnt the flow rate of the rupture disk should be the flow rate in the protected line? What is PM that is mentioned in your review?

 

[CHD01]

I am going to make the assumption that the only source of overpressure to be considered in your installation is dead-heading the pump; and that liquid thermal relief and other relief scenarios are not applicable. If this assumption is incorrect let me know.

With that said, you need to determine the MAWP for the pipe system installed and then look at the pump curve to determine the max pressure at ZERO flow (if ZERO flow is really possible). In looking at the curve you need to look at flow conditions at required relieving conditions (usually 10% over relieving pressure); this will result in a flow less than normal flow since the pump head is increased. If this results is a deadhead pressure greater than the MAWP of the pipe system, then you need a relief device UNLESS you can limit the impeller diameter to prevent overpressure. You may also be able to reduce flow based on the limitation in HP provided by the pump motor. Last, if there is a minimum flow return line for the pump (from pump discharge to its suction system or tank) this will also reduce the required maximum relief flow.

If the piping distribution network is such that only a certain percentage of the total flow can be blocked off from any one valve being closed; then you only need to relieve the excess flow resulting in the excess pressure.

The bottom line is that NO - IT IS NOT AUTOMATIC THAT YOU MUST RELIEVE THE TOTAL FLOW AT NORMAL CONDITIONS. This is a conservative first approach, but with a huge flow such as you have it needs a more detailed engineering analysis.

There are lots of relief issues here that I am not covering, but perhaps this will get you started.

USing rupture disks for this application sounds strange - remember this device cannot reclose and you will release a large quantity of liquid and you will have to shut the pump down unless you can valve in a spare disk while valving out the failed disk. But like I said there is so much more we could talk about and I know so little about your system.


Good Luck!
The more you learn, the less you are certain of.

 

[greg87]

CHDO1 did a good job on answering your follow up question, as well as identification of the key issues involved. PM in my first response is for Preventative Maintenance.