| Larry Bachus
(a.k.a. "The Pump Guy")
Hello Pump Guy,
I’m a process engineer at a steel mill in India. We have a problem with our cooling tower pumps. The main pumps can generate a maximum pressure of 9 bar (gauge), but this is apparently not enough pressure. Too much pressure is lost across the cooling water circuit. It is difficult to push the outlet water into the outlet header. To overcome this problem we are proposing adding a booster pump into the outlet circuit. Actually, we propose two booster pumps, a primary and a standby.
Kindly advise if this proposal is technically viable. Some drawings are attached.
Adding a booster pump is technically viable when the system demands it. If the original cooling tower loop contained 8.5 or 9 bars (gauge) of energy, the cooling tower pumps should develop 9 bars (gauge) best efficiency pressure. If a recent system modification (new filters, valves, heat exchanger, longer pipe runs, etc.) consumes additional energy, then a booster pump is a potential solution.
I really need more information to offer an opinion. I’ll offer some thoughts, and you may answer your own question at the end of this communication:
- Is your cooling tower pipe loop under construction?
- Is your cooling tower pipe loop recently commissioned?
- If not new, how old is your cooling tower pipe loop?
- If your cooling tower pipe loop has existed for many years, do the original engineering drawings match the existing pipe scheme?
The schematic you provided doesn’t show:
- Clogged filters and strainers in the loop, or
- Normally throttled control valves, or
- Pipe scale. A 200 mm (diameter) water pipe eventually becomes a 170 mm pipe and then 140 mm diameter. The additional friction and velocity losses may be the real energy-consuming culprits leading you to consider a booster pump.
We can’t see long-lost pipe wrenches and “come-alongs” lodged in the piping system elbows. The drawings don’t show new equipment (probes, flowmeters, equipment substitutions, etc.) added since the cooling water system was designed and commissioned.
We can’t see the “spring load” or “weighted arm” on check valves or if the loads were altered or changed.
We can’t see the NPSH available to the pump suction nozzle. This can be measured with a gauge.
The schematic doesn’t show the actual installed impeller diameters or the actual motor speeds for comparison with the original specs. The impeller diameter, wear-ring tolerance and speed are big factors in determining if the pump can meet the demands of the system.
The main pump may require a 300 mm (diameter) impeller. But, after many years of service and many trips to the shop, the impeller diameter can be less than 300 mm. This will affect the main pump’s developed head.
In India, you have 50-Hz electricity. The original pump might have been put into service with a four-pole motor rated at 1,485 RPM. If the actual motor speed is only 1,420 RPM, the main pump’s discharge head is reduced by the square of the reduction in the speed. And, you are considering booster pumps.
The drawings don’t show the system curve mated to the pump curve. Where is your pump operating on its performance curve? Any of the above conditions and modifications can make you think you need booster pumps.
Think of it this way: Your car has a fuel filter between the fuel pump and the carburetor. If the fuel filter clogs with debris, the fuel pump’s energy is lost into the fuel filter. The starved engine will spit, sputter and stall. Your car won’t accelerate or perform properly.
Do you need a larger fuel pump with more power? No! Do you need a booster fuel pump? This won’t resolve the problem either. Why not change the clogged fuel filter and enjoy your car’s performance?
Now, think about your cooling tower pumps. Is it easier to install booster pumps? Or, is it easier to locate and correct the energy thief?
Locating the problem is relatively easy. Review the following scenarios; these will plot a course of action.
1st Scenario: If your main pumps and the cooling tower loop is completely new, this is a problem for the cooling system manufacturer. Something is out of control.
Suggested action: Let them solve it. Contact the manufacturer, read ’em the “RIOT ACT.”
2nd Scenario: If your relatively new cooling water system is “Out-of-Warranty” and the pumps never properly delivered head and flow, then it’s your problem, but it is easy to fix.
Suggested action: Contract a pump specialist in your city and work with them; or I’ll come to your plant if you can’t find someone locally.
3rd Scenario: If your cooling water system has been functioning correctly for many years and suddenly has new demands on it, then you may be able to upgrade your existing main pumps or consider the booster pumps.
Suggested action: Decide if you need more head, or more flow, or more head and flow to meet the new demands on the system. Get quotes on a pump upgrade or larger pumps. Get quotes on higher speed drivers. Compare the cost and installation of booster pumps to the cost and installation of other options.
4th Scenario: If your existing cooling water system has performed correctly for a number of years, but has mysteriously stopped functioning correctly, you must locate that mystery and correct it. The mystery will reveal itself with some well-placed pressure gauges and a clip-on flowmeter.
Suggested Action: Contact me or a local pump specialist. Or, read Chapter 8 of my book, Everything You Need to Know About Pumps.
Start with the pump (performance curve, gauges, flowmeter) and confirm if the pump is correct or incorrect. If the problem is not the pump, it’s the system.
The energy (head) designed into the pump should be the same as (or very close to) the energy contained in the system. So, you can see that booster pumps may be the correct route to solving your pump (or system) problem.
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 retired member of ASME and lectures in both English and Spanish. He can be reached at firstname.lastname@example.org.