Cooling Towers and vertical pumps 1

[arm83]

Hi people,
I am new in the industry and have to justify replacing five horizontal pumps with three vertical ones. Let me expand.

These five horizantal pumps have been troublesome for the last 30 years with lots of bearing and seal failures and shaft breakage. These pumps are double suction worthington-simpson 10LA4. The problem has been determined to be poor piping.
Anyways, the idea is to replace these five pumps with three vertical, wet-pit submersible pumps. (ie just the suction, not the whole pump, would be submerged)
What I would like to know is what are the advantages of vertical pumps over horizontal pumps?
Thank you in advance for your help.

 

[BigInch]

See,

Horiz. split case v. vertical turbine
thread 164-80905
thread 407-167849

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[pennpiper]

I think vertical pumps are better for a lot of reasons and they take a much smaller "footprint" (space) than the horizontal pump.

The suction side of the vertical pump does not need any piping or block valves.

You may have a problem converting from horizontal pumps to vertical pumps. The existing suction basin was/is designed for the existing horizontal pumps. Vertical pumps normally require a different basin design than a horizontal pump.


Note to others who may need to design a large cooling tower installation with concrete a basin.
1. If your tower basin is in ground and your horizontal pumps are at or about ground level then the horizontal pumps need a dip legs on the suction and are working very hard to draw the water.

If your tower basin is in ground and your horizontal pumps are in a below grade pit along side the suction basin then you do have short level suction pipes but you need block valves in order to shut-off the suction side and work on a pump. This design also needs a very good seal on the suction pipe wall penatration.

 

[Artisi]

I would think that 30 years of bearing, seal failures and shaft breakages in conjunction with time and money involved in repairs would be more than enough justification to investigate what the problem is and not necessarily look at a different pump style.

In all fairness to the pumps, the failures may not be a fault of the pump but more likely the application -- will installing a different pump type overcome application problems - more than likely not.

I would suggest that your first priority would be to investigate WHY the existing pumps are failing before rushing off to justify a major change to your installation

 

[JJPellin]

I have never heard of using a vertical wet-pit pump in a cooling tower installation. We have five towers and all but one of them only use only vertical pumps. But all of the vertical pumps are single stage vertical turbine pumps. Some of these pumps are trouble free but many of them are very unreliable. We have only one horizontal pump in cooling tower service. It is a double suction Worthingon that is older than our refinery. It is by far the largest and by far the most reliable. I have worked here for 18 years and we have had only one overhaul of this pump during that time. You mentioned piping as the likely cause of the poor reliability of the existing pumps. I would wonder if there is a horizontal elbow in the suction line causing an imbalance of flow to the two impeller eyes.

Vertical pumps can be very reliable. But you need to be careful to be certain they have adequate NPSH, adequate submergence, a properly designed sump and are running above the required minimum flow based on suction specific speed.

 

[ccfowler]

arm83,

Artisi, JJPellin, et al, have given you the best suggestions that I can imagine. The essence of engineering is recognizing that there is seldom, if ever, a single right solution to a problem. The important issue is to arrive at the most reasonable set of compromises that serve the needs of the particular application.

Your statement, "I ... have to justify ..." is very familiar to me, and it is associated with numerous disasters or potential disasters, and always associated with misapplication of pieces of major equipment (not just pumps). I will not go into an "old geezer rant" about this, but do take caution.

The pump type, size, drive, controls, ... are always best determined by the needs of the specific application and never by a "need to justify."

Since your application has a three decade history of difficulties, it would be wise to reconsider the pumps and piping for your application in light of current pump application knowledge and practices that may highlight the source(s) of your pump's problems. Don't be surprised to find that cleaning up piping issues solves your pump problems. Also, you may find that adding adjustable speed drives may help to permit the existing pumps to function more suitably.

From my experience and observations, pump problems almost always arise from their associated systems, mis-application, or mis-operation rather than from an inherent flaw in the pump itself.

 

[Artisi]

ccfowler --"From my experience and observations, pump problems almost always arise from their associated systems, mis-application, or mis-operation rather than from an inherent flaw in the pump itself"

I can concur fully with your statement - how often do you / have you heard "there's something wrong with the pump" this is not only heard in the field but also in this forum.

Why is it it's always the pump - seems the logic is - when the pump is stationary there is no problem once the pump is running we have a problem - therefore it must be the pump.

Luckily we learn after awhile that the pump usually comes near the end of the check list when you have to analyse a pump application problem.

 

[arm83]

All
Thank you for your responses,and I will keep them in mind.
One of the recurring ideas in most of the responses is to analyze why the horizontal pumps have had such a high failure rate over the last thirty years. JJPellin hit the nail on the head with the the horizontal bend before the suction of the pump. A report was written about it before I started working here and a recommendation was made to change the piping but there was never any follow through and the failures continued to occur. It does not make sense though (at least to me, please correct me if I'm wrong) to keep these pumps as they have been rebuilt so many times, the specs are no longer OEM and the casing itself has been eroded.
As to replacing them with vertical wet pit pumps, these orders come from higher up and one of the major justifications is that these pumps do not need to be primed.
On another note, could someone explain how NPSHa or/and NPSHr varies with a vertical pump as opposed to a horizontal one ? From the threads Big Inch supplied, it was mentioned that NPSHa is reduced ?
Thanks for your help guys.