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September 2006
 
  Mailbag: Suction Pressure Problems
Using Instrumentation to Reveal Energy Robbers 		 
   
 

Larry Bachus
QUESTION:

Hi Pump Guy,

I am the maintenance engineer at a power station in Zimbabwe. We have a problem with one of our pumps. This pump is one of six pumps in parallel with a common suction and discharge line. When the problematic pump is operating alone, its suction pressure will be at 28 kPa. When there are two or more pumps running in parallel, the pressure rises to around 38 kPa. We can’t explain this. Why? What affects the suction pressure?

Your assistance will be greatly appreciated. I can furnish more specifics if needed.

Regards,
Ladislous - Mech. Engineer


ANSWER:

Hello Ladislous,

You say the available suction pressure is lower (28 kPa) when only one pump is drawing on the liquid than it is with two or more pumps (38 kPa) in parallel drawing on the available liquid. This does seem illogical on the surface, unless there is a reason for the illogic.

Let's consider the NPSHa, or the suction energy in the liquid available to the pump (or pumps). The NPSHa is independent of the suction head required (NPSHr).

The answer may lie in a suction restriction, like a check valve. One pump's requirements may not be able to generate enough differential pressure to adequately open the check valve. Thus, one pump may be starved (lower suction pressure available to just one pump). But, two or more pumps drawing on the liquid across the check valve may cause adequate differential pressure to completely open the check valve and feed both pumps (higher pressure available to multiple pumps).

This is just one possibility to justify the apparent illogic. Most people are aware that the automobile air conditioner cools better at higher speeds than in slow traffic even though the AC fan motor speed is constant. The reason is because at higher speeds, the differential pressure and velocity is greater across the cooling coils.

All restrictions, like check valves, function based on the differential pressure (or head) across the restriction. I can't see your system, and even if you sent me drawings and schematics, I can't see closed valves and other obstructions (spring loads and weighted arms on check valves, clogged filters, pipe schedules, or even a wrench lodged in a short radius pipe elbow) that might act on the pump(s). You can't see these obstructions (energy robbers) either. So I suggest the following, which you should find economical, interesting, and professionally satisfying.

Install a few pressure gauges throughout the suction piping of your system. Put your first gauge at the discharge nozzle of the previous process or tank in your piping system. Install other gauges before and after all obvious obstructions (elbows, tee-joints, filters, valves). Finally, install your last pressure gauge at the suction nozzle of your pump(s).

Be sure the gauges are accurate and calibrated. Bleed and burp any possible or suspected air pockets in the suction piping. This assures accuracy. Record all pressure gauge readings and note the differentials when the system is off and the pumps are not running. Note open and closed valves.

Next, start one pump, allow the system to stabilize, and note the further differential-gauge readings throughout the suction piping. Then add a second and third pump one by one, and note the further differential-pressure drop across the suction pipe.

The differential gauge readings (upstream to downstream) across the system (with pumps running compared to off) will reveal the friction and velocity losses in the system and tell you where and how the energy is being robbed from the suction manifold and distributed to the pumps. The illogical and will soon become logical.

Larry Bachus - “The Pump Guy”

Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a member of ASME and lectures in both English and Spanish. He can be reached at larry@bachusinc.com or 615 361-7295.

www.bachusinc.com

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