Discharge side check valve for centrifugal slurry pumps

[cokeguy]

We were told today that we should use check valves on the discharge side of our slurry pumps to prevent possible (major) damage. We have relatively large (6,8 and 10 inch discharge diameter) pumps, discharging to 10, 12 and 16 inch diameter 30 feet high discharge manifolds. The calcium sulphate and calcium carbonate slurries we use are higly abrasive, corrosive, and tend to stick everywhere, so I assume that´s the reason why check valves weren´t installed in the first place (we do not even have discharge side valves in some of the pumps, but that can be solved easily). Are we really risking major damage to the pump in case of, for example, the impeller back-turning after a power failure because of the 30 feet discharge side water column, and somebody turning the pump back on while it is still backturning? These guy who recommended the check valve supplies the mechanical seals for some of our pumps, and he says that even the seal could be damaged by the water hammer (?) effect. Any comments will be very welcome. Thanks..

 

[MMcLean]

In my opinion check valves in slurry service are almost a waste of time. Can your budget accommodate actuated discharge on/off valves and logic to close the valves when the pump stops?

Mark McLean

WorleyParsons

 

[stanier]

The advice you have received could be good advice. I say could be because you need to analyse the system for surge on power loss. The seals and bearings are at risk if there is a high presure surge.

Any analysis should be part of a risk management assessment. What is the likelihood of a power failure or hitting of an emergency stop? What are the consequences of losing one pump? DO you have a standby?

If you go to

http://www.pipingdesign.com

there are two papers there on surge. You will find my contact details on them.


If you need a check valve to protect the pumps the Duckbill type by RED valve are a good but costly selection for slurries. They will cope with very abrasive slurries.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[JJPellin]

There are a few possible reasons to need a check valve. Many ANSI pumps have impellers that are threaded on. The common argument is that the impeller will unscrew, drive itself into the case and damage the case and impeller if allowed to spin backwards from reverse flow. This is not very likely. The torque impossed on the impeller to spin it backwards would tend to tighten the impeller, not loosen it. For API pumps, the impeller would be key driven and should not unscrew. Most pumps would not necessarily suffer any mechanical seal or bearing problems from running in reverse. Oil rings work in reverse. API plan 11 or 13 seal flush might not flow at normal rates with reverse flow, but this would tend to be a long term risk, not an immediate one. The primary reason we install check valves in the discharge of our pumps is not to protect the pump, but to protect the process. If a pump trips, the second pump will often be set up to autostart. If there were no check valves, the back flow through the tripped pump would divert all available flow from the process and shut the unit down. If the pump pumps fluid from a low pressure tank to a high pressure vessel we need to protect against back flow that could over pressure the supply tank. If you don't have a process that would justify a check valve, it is probably just something else to fail. We don't use check valves in liquid sulfur applications for this reason. No check valve would be reliable and it is not needed to protect the pump or process.

 

[Artisi]

The first question/s I would ask myself is;
1. How long have we operated these pumps without check valves on the discharge?
2. Have we ever experienced any problems?
3. What happens now when you shut-down the pumps under normal operating conditions?
4. Has there ever been any water hammer?
If you have never experienced any problems;
5. Why change the operating system now?


Furthermore, having the pump driven backwards by flow from the discharge will NOT unscrew the impeller.
If there is a chance of the pump starter being activated while the pump is still running backwards, simply put a delay timer in the control system set for a measured delay period.

Naresuan University
Phitsanulok
Thailand

 

[cokeguy]

Like JJPelin states, I really wouldn´t like to add another possible failure element to the system (either check valves or automatically actuated discharge on/off valves) without proper justification. My main concern right now is catastrophic pump failure because of a backturning impeller. We don´t have pumps operating in parallel, no concerns about that, and if the pump stops because of power failure or something like that, we simply shutdown the process, we can live with that while we start up the system again. We have toyed with the idea of automated discharge valves, but if a valve fails and accidentaly closes during operation and the operations guys don´t react swiftly, THEN we risk major damage to the process equipment because of increased temperature. Up to now, we´ve been working ok for 3-4 years, and have had a lot of shutdowns, power failures, etc. with the corresponding backturning pumps after a shutdown, and no problems whatsoever. However, when I heard this guy yesterday talking about possible catastrophic failures because of the lack of discharge check valves, it made me think if we should do something about it. I can understand the stresses involved if you turn on a pump while it is backturning after a stop, but that could be easily remedied by a timer within the control logic, as stated by artisi.

What is not very clear to me is how could a significant water hammer effect happen in my system if a pump shuts down and backturns for a few moments until the discharge column is depleted. Our system is pretty straight forward, tank and pump are both at floor level, and have a simple 30 foot high discharge manifold which discharges via control valves to a scrubber at atmospheric pressure levels, no other high pressure generators involved. I read the papers suggested by stanier (thanks by the way stainer, very good references), but I still can´t visualize a significant water hammer in my specific case. Any references that talk about water hammer effects in simple pump applications with a not-to-tall discharge manifold such as ours? Thanks again.

 

[stanier]

Hi Cokeguy,

Have you experienced any waterhammer effects in your system? Normal SCADA instruments do not have fast enough transducers to pick up pressure transients.

In respect of slurry pumps with hard iron impellers these are fixed by threading the impeller on the shaft. I have seen the impeller screw off and jam against the pump housing. But that was because the spaarky had wired the motor the wrong way and the commissioning engineer didnt check the direction of rotation before fitting the vee belts and aligning the pulleys. I have heard of pump impellers unscrewing due to water hammer but these have been on long pipelines that reverse flow through a forward rotating pump where the check valve had failed.

There are two types of damage from waterhammer. One is catastrophic-linebreak, pump impeller coming off. Risk assessment possibility =??? Consequences=???? Your call.

The second is repetitive fatigue damage. Likelihood based on number of stops=???? Consequences of losing one pump so it can be fixed depends on having a standby or not and how busy your maintenance people are, increased maintenance, cost of spares=??? Again your call

Paraphrasing the Mastercard advertisement in Australia at the moment:-
A surge analysis of your system for one pump would cost= < $2000.
Cost of check valves would be = $4-16000 per pump + installation. Even more if production is lost.

Cost of a good night's sleep and not losing your job= Priceless.



Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au