12 replies to this topic

[hannkb]

Hi there ...

I need some help in setting the trip point for an OWS sump pump. The pump is to trip when it reaches low discharge pressure.

What do I need to have? and what do I need to do with what I'm supposed to have?

I'm a newbie engineer and would really appreciate your help in completing this task.

Thanks

[Art Montemayor]

hannkb:

A sump pump is normally set to pump out a sump level and, as such, is set to trip out on low-low sump liquid level (for either the bottom or top liquid, should the sump be acting as a decanter or oil-water separator.

I have never come across a sump pump that trips on low discharge pressure. As I said, the sump pump is nothing more than a "scavenger" type of pump that is used to move liquids below grade to a more controlled or managed environment and, as such, there is no importance given to the discharge pressure as long as the liquid transfer is successful.

Are you definitely sure about setting a low discharge pressure as the trip-out set point? Please explain the scope of work for this operation or furnish a detailed drawing.

Await your reply.

[Qalander (Chem)]

I second Art since at my previous employer we had 'level based trip at oil/water separator'. Nothing to do with discharge pressure.

Regards
Qalander

[hannkb]

Gentlemen,

I really appreciate your input. What you are saying is absolutely true. However, our company has suffered from many sump pits incidences because of running the pump dry and having flammable HC waste gathered inside them. So the decision was made to make like a double-contingency approach in which the pump is to trip on low discharge pressure in addition to the low-low level criterion. This criterion would be applied when the pump get plugged due to dirt accumulation inside the sump and the level is no longer an indication for running the pump dry. Again, it's a double contingency. In fact, the low-level trip point wasn't efficient because of poor level transmitters, so we are now installing guided wave radar transmitters in all of our sumps.

The problem with the design of our sump pits is that we don't have a standardized design with regards to the disposal of HC to the sewer rather than a closed pressure system.

Please let me know if more info is required.

[fallah]

In fact, the low-level trip point wasn't efficient because of poor level transmitters, so we are now installing guided wave radar transmitters in all of our sumps.

If there is the possibility of pump getting plugged due to dirt accumulation inside the sump leading the pump to work dry,the problem who to be solved by changing the level transmitter?

[hannkb]

Dear fallah,

thanks for your reply. Yes, we are now changing the level transmitters, we are installing GWR "Guided Wave Radar".

But as further protection, we want the pump to trip when it reaches low discharge pressure.

We can't afford more 36 incidences "16" of them were explosions because of having the pumps running dry.

So my question is, how do I set this point? the low discharge pressure?

[fallah]

Dear fallah,

thanks for your reply. Yes, we are now changing the level transmitters, we are installing GWR "Guided Wave Radar".

But as further protection, we want the pump to trip when it reaches low discharge pressure.

We can't afford more 36 incidences "16" of them were explosions because of having the pumps running dry.

So my question is, how do I set this point? the low discharge pressure?

I think with the possibility of pump suction to be plugged,using LFA (low flow alarm) and trip in discharge line would be better because of simultaneously avoiding pump running in blocked discharge (specially in the case of no minimum flow line).The relevant set point could be the minimum flowrate submitted by vendor.

[Qalander (Chem)]

Sorry, I was out on some other business,however you have lot of good tips from our learned colleagues/forum members.

One small query that do you have some basket strainer of optimum mesh size to minimize the dirt plugging of pump suction/impeller?

Best Regards
Qalander

[hannkb]

Thank you all gentlemen for your valuable contribution.

I like the low-flow alarm, as it will be protecting the pump in one more case which is the blocked discharge case.

So you guys believe, for our case, low-flow alarm approach is safer than the low-pressure alarm?

[djack77494]

So you guys believe, for our case, low-flow alarm approach is safer than the low-pressure alarm?

I would weigh in on the "I prefer using low flow" side of this issue. That's because, as you said, it would protect against a blocked discharge scenario. Also, I don't know how an extreme "dirt buildup" would impact the discharge pressure, but seeing a drop in flowrate would be a good indication of a problem. In general, prefer the more direct approach - If low flow is the problem, prefer a low flow alarm.

[Art Montemayor]

hannkb:

You have not responded to my request for: "the scope of work for this operation or furnish a detailed drawing".

As such, I can't (nor can anyone) give you a definite, accurate evaluation or comment. All I can do is guess.

And my guess is the scheme(s) you and your company propose will not work without costly and very complicated instrument controls (maybe a PLC). My guess is based on pure logic and common sense:

1) A sump is an open pit designed to catch impromptu or sporadic, unsteady-state flows from various points and, as I previously stated, "is used to move liquids below grade to a more controlled or managed environment". That is why the sump is a cheap, non-pressure receptacle. Common-sense dictates that since the flow is not steady, the pump will always pump the sump down to its lowest acceptable level AND SHUT DOWN. That is the function of a sump pump; it is designed NOT FOR STEADY STATE FLOW, but for intermittent stop-and-start operation. Therefore, when the pump shuts down because it has pumped the sump down to minimum level, WHAT WILL BE THE FUNCTION OF THE LOW-FLOW SWITCH? The pump is "tripped out on low-flow", so you wind up with redundant instrumentation.

2) Since the pump is now tripped out and there is ZERO FLOW, what now? When the flows to the sump resume and the level rises, the pump is maintained tripped out due to NO FLOW. A decision has to be made for the pump: when does it start up and when will it shut down - considering that the level and flow (& pressure conditions) may be contradicting each other as far as positive signals to the pump instrumentation?

Unless we have what I requested, any advice to you on this subject is (in my opinion) irrelevent and only good or bad guesses as to what is going on.

The practical and common-sense cure or solution to dirty fluids that carry sludges, fines, or solids that tend to clog up the sump pump is to build a deeper sump - allowing the solids and sludges to precipitate to the bottom and subsequently pump only the decanted liquid at the top. Judicious cleanup of the solids with a sludge pump (as in a septic tank) or manual maintenance keeps the sump operating 100% of the time. This is the time and field proven method of handling this type of bothersome pumping operation.

Common sense also dictates that if you have plugging in the pump casing (where there is the highest turbulance), than YOU WILL SURELY EXPERIENCE PLUGGING OF ANY PRESSURE SENSING INSTRUMENT YOU INSTALL IN THIS SYSTEM. By pure logic, a pressure switch in this application is a naive and failure-prone recommendation subject to failure.

Sump pump operation is not Rocket Science. We old, Jurrasic Era engineers have been dealing with the problem since time immemorial - and making it work with only a commode-type of float switch. It's that simple; although a microware level detector is a great instrument improvement.

[jr31]

The trail seems to have gone cold on this issue.

I have designed instrumentation for Oily water sumps and have gtenerally used a magentic flow meter with low flow alarm/interlock for very reasons( pump clogging). In few instances I have also used pressure transmitters/Pressure switches on discharge to alarm/trip the pump. The set points I have used was 10% of nominal flow rate or 10 % of expected discharge pressure expecting that these are safe enough values to give alarm or trip without damage to the pump. Some of the systems are not yet commissioned. ( the one with pressure has been commissioned & reported to be working fine). Hence a feedback will be very useful.

In addition I am proposing Coaxial GWR transmitter with so called "large diameter probes" but like to know experience on ROD type probe , (or any other used by others)As rightly mentioned in discussion DK cannot be properly guessed and hence I work on basis of DK= 1.4 (Minimum).

In some applications we have a good experience of using a Non contact radar(Process) for same applications.

Finally In case of some sumps the approach has been to use two transmitter , one GWR, one Non contact with either a Flow or Pressure transmitter.

Yes looks like lot of instrumentation! but Know of Explosions in in pit and therfore think this investment worth done!

Feedback on this will be highly welcome!

Thks

Hi there ...

I need some help in setting the trip point for an OWS sump pump. The pump is to trip when it reaches low discharge pressure.

What do I need to have? and what do I need to do with what I'm supposed to have?

I'm a newbie engineer and would really appreciate your help in completing this task.

Thanks

[jr31]

The trail seems to have gone cold on this issue.

I have designed instrumentation for Oily water sumps and have gtenerally used a magentic flow meter with low flow alarm/interlock for very reasons( pump clogging). In few instances I have also used pressure transmitters/Pressure switches on discharge to alarm/trip the pump. The set points I have used was 10% of nominal flow rate or 10 % of expected discharge pressure expecting that these are safe enough values to give alarm or trip without damage to the pump. Some of the systems are not yet commissioned. ( the one with pressure has been commissioned & reported to be working fine). Hence a feedback will be very useful.

In addition I am proposing Coaxial GWR transmitter with so called "large diameter probes" but like to know experience on ROD type probe , (or any other used by others)As rightly mentioned in discussion DK cannot be properly guessed and hence I work on basis of DK= 1.4 (Minimum).

In some applications we have a good experience of using a Non contact radar(Process) for same applications.

Finally In case of some sumps the approach has been to use two transmitter , one GWR, one Non contact with either a Flow or Pressure transmitter.

Yes looks like lot of instrumentation! but Know of Explosions in in pit and therfore think this investment worth done!

Feedback on this will be highly welcome!

Thks

Hi there ...

I need some help in setting the trip point for an OWS sump pump. The pump is to trip when it reaches low discharge pressure.

What do I need to have? and what do I need to do with what I'm supposed to have?

I'm a newbie engineer and would really appreciate your help in completing this task.

Thanks