Pump Dead Head Pressure 2

[Jeremy H]

Hi all,

We have a situation at one of our manufacturing plants. The progressive cavity pump has pump protection installed to prevent the product from being over-pressurised. The high pressure trip is 14 bar and the burst disk is rated to 20 bar. The issue that is taking place is that when the pump is dead headed (valve downstream closed) the burst disk ruptures first (less than a second after the pump starts) and then the pump trips on high pressure (if it does trip). My theory is that the burst disk is rupturing due to a hammer effect through the pipe. I want to calculate the time it takes for the pressure in the pipe to reach the burst disk rating to validate this theory.

Details:
- Operating flow rate is 210 kg/min
- Density of product is 1330 kg/m[sup]3[/sup]
- Normal pump operating pressure is 10.5 bar
- Diameter of pipe is 80mm
- Length of pipe from pump outlet to valve is 15m
- Length of pipe from pump outlet to burst disk is 0.25m
- Motor is 6.82kW
- Assume all components are on the same vertical plane

Any assistance is much appreciated. Let me know if you require any additional information.

 

[LittleInch]

If your pump protection is simply a high pressure switch for what is quite a large motor then I am far from surprised you have this issue. The amount of fluid you need to overpressure the pipe will equate to less than one second of the pumps capability for 15m of 80mm pipe. It's nothing to do with surge or "hammer" effect, simply the pump is doing what it is designed to do - move fluid - into a closed volume. As your fluid is basically incompressible, as you try and force more fluid into a small fixed space, the pressure will rise very very fast.

If you really want to, its easy to work out how much fluid is required to raise the pressure from 0 to 20bar and then back calculate how many milliseconds of flow this equates to. Personally I wouldn't bother - it's your system that needs changing.

This sort of pump should simply not be allowed to start against a closed head or it needs a pressure relief valve set at some suitable value to relieve the pressure back to the tank or the pump inlet.

Installing a positive displacement pump without a pressure relief line before the first isolation valve is simply very poor design ( based on the limited information you've supplied).

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bingopin]

That is an excellent, thorough reply. Star for you.

 

[JJPellin]

I would suggest adding a discharge accumulator to the system. You can size the accumulator to hold enough volume to allow the high pressure trip time to respond and shut the pump down before the rupture disk pops. Based on the instrumentation and controls in your system, a couple of seconds should be long enough.

Johnny Pellin

 

[georgeverghese]

Agreed, a recycle valve, which may be a control valve operated by PIC or a pilot operated backpressure regulator set at say 12barg or so should help to prevent the RD from bursting for cases where the downstream valve closes slowly enough for this recycle valve to respond, and for the sunsequent PSHH to bring the pump to a dead stop. If the downstream valves closes fast, a gas charged discharge bottle ( dampener or accumulator with an internal elastic diaphragm) will be necessary to buy some time for the recycle loop to react, and for the PSHH to stop the pump.

Slowing down the closing speed of this downstream valve will also help, and may avoid the need for this dampener. If this downstream valve is air operated and fail close, install a speed controller device on the actuator to slow it down.

The internal relief valve built into some models / makes of these pumps may not be of much use in this application, since you have limited head room between 10.5barg and the lowest possible RD burst pressure. They usually start to crack open at 75-80% of set pressure.

 

[Jeremy H]

Hi all,

Thank you for your responses. I will take on board all of your suggestions / considerations.

I agree that the system isn't the best designed. The reason I'm trying to determine the time it takes to reach burst pressure is so I can potentially use this number to prompt a more detailed review. I want to verify whether the time it takes to reach burst pressure is significantly quicker than pump protection (high pressure) response time.

There have been previous instances of dead-head not related to valve closure where the product has crystallised and caused a blockage in the static mixer. Not that I'm ignoring the issue with the valve closure but there are other possible causes of dead-head. I can't recall seeing any pressure relief line in the industry that I am in. Usually the acceptable standard is to just use a rupture / burst disk (doesn't mean that this is sufficient reason to not use a pressure relief line).

LittleInch, you mention that it is easy to determine the quantity of fluid required to increase the pressure from 0 to 20 bar. What equation / theory should I be looking to use? I don't have the strongest background in fluid mechanics so I would appreciate a nudge in the right direction. You mention that it will take less than a second to over-pressurise the pipe. Is this based on information that I have provided?

 

[LittleInch]

Simplistically use bulk modulus

http://www.engineeringtoolbox.com/bulk-modulus-elasticity-d_585.html

In your case I don't know, but lets use water, BM of 2 x 10^9 Pa (round numbers)
Using 20 bar, (2 x 10^6Pa), the change in volume to get an increase of 20 bar is approx. 0.001
Your 15m long 80mm tube holds 0.0753 m^3, so the volume required is 0.0000753m3
Your pump does 0.00263m3/sec
SO the time to pump the volume needed to gain 20bar is 0.0000753 / 0.00263 = 0.002 secs.

Now your pump is winding up and the pipe might expand a bit, but even so you're going to struggle to get beyond 0.1 second.
This will also set up a bit of a shock wave as well, but that's minor compared to the pressure from the volume increase.

You have quite a decent size motor on this so it will provide a big instant force.
To be honest I don't know if a relief valve will be able to act that fast either.

I would just inhibit pump operation if the downstream valves are closed

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

It is likely the pressure would reach 20barg even as the downstream valve approaches fullclose position. Control valve capacity calcs will tell you at what intermediate position the pressure upstream of the valve would reach this pressure whilst still flowing at 210kg/min. So it would have nothing to do with liquid hammer. This event will happen even earlier if the RD ruptures at say 90% of set pressure.

 

[Jeremy H]

This has all been very informative. I believe I have enough information to prompt a more detailed of our systems.

Thanks to everyone for their assistance.

 

[pumpingtips]

Bursting disks are not a good solution in many of our simple PD pumping operations. There is normally no great potential for harm to people. Relief valves are better solution. No valves on these lines is best. Interlocking of valves with limit states is required. Do not start pumps until valves are fully open. In the case of blocked lines your relief valves will activTe