This article discusses the performance problems associated with process water pumps. It highlights that at a boiler steam load in the 25% to 40% range, an automatic recirculation (ARC) valve is simply not accurate enough to detect a minimum pressure differential change of possibly 1–2 psig when the boiler drum level control valve closes for short-duration process demand changes. Converting the ARC valve to an electronic controller does not work reliably either. The amperage change is difficult to discern at low feedwater flow rates. Failure of the valve to open is a recipe for pump cavitation and becoming “steam bound.” If the boiler trips on “low water,” plant production can be affected immediately. For larger boilers with high-capacity, high-horsepower feed pumps, the ARC valve often becomes an energy conservation (cost) issue.
During a visit at a large industrial facility in the Southeast I became involved in a technical issue that could perhaps be referred to as part of a Centrifugal Pump Operations Session 101.
As a staff technical leader in the corporate utilities group for a pulp and paper company, I had been scheduled to visit a tissue mill to conduct combustion efficiency testing on two steam boilers.
During a preliminary inspection of the boilers and auxiliary systems, I noticed that both of the 100 hp boiler feedwater pumps were running. This appeared odd to me.
The boilers were installed as a 100 percent redundant system. One boiler was generally in service and the second unit was in warm stand-by. The feedwater pumps were also 100 percent redundant. At a steam load of 90,000 pounds per hour (pph) there should have been no reason for two pumps to be operating. Both pumps seemed to be running OK; so, I went to the control room to further check out the distributive control system data and have a discussion with the shift utilities crew.
An operator said both pumps had been in service for several months. A year earlier, I had inspected the pumps and suggested a minor overhaul because of their age. The pumps were two-stage, horizontal, split-case, double-suction design. It is an easy, straightforward repair procedure: Remove the casing upper half, check the impeller eye, the wear rings, and the casing cut-water for erosion, and then repack the glands if everything looks normal.
The boiler operator said maintenance had overhauled the pumps during the previous summer’s outage and had found nothing abnormal other than worn packing.
Plant steam pressure was 215-225 psig and feedwater pressure was normally 320 to 340 psig with one pump in service. Each pump was rated for 290 gpm at 720 feet of head. The deaerator was being operated at 7 psig (233 °F); however, it was installed on the roof of the boiler building with a pump suction elevation head of 45 feet. No way should cavitation be an issue with these pumps.
I asked the operator why they needed to run both pumps. “One pump could not maintain boiler drum level and header pressure during sudden steam load changes,” he replied. Wow! Two pumps in parallel should double the feedwater flow or increase the pressure to near a maximum shut-off head of 360 psig at half flow rate, I thought.
My next action was to conduct another inspection and gather more data. Both pumps were pulling 82 to 84 amps (about 75 percent of motor rated load).
Feedwater flow certainly appeared high. Blowdown valves were closed and cold, as were the bottom header drain valves. We seemed to have a combined feedwater flow of about 440 gpm for a 70 percent boiler load of 90,000 pph. Where was the additional 200-plus gpm of feedwater going? Each pump (operating at 220 gpm) was about 53 percent efficient. Together, they were using a total of 148 hp.
Both pumps have a 1-inch discharge recirculation line that provides a minimum continuous flow of 10 percent (about 30 gpm in this case) back to the deaerator to assure against a “no flow” condition inside the pump when the steam drum level control valve goes automatically closed.
Since the feedwater is 233 °F, these lines are fully insulated and can be difficult to discern. It took me a few minutes to locate the orifice-plate assembly about three feet above the pump casing. The aluminum jacketed lines were about 3 inches in diameter, and some of the insulation appeared to be new. A quick sound test, with the pointed end of a screwdriver held against the recirculation line pipe and the handle end against my ear, produced high- frequency noise, an indicator that the flow velocity noise in the pipe is much higher than reasonable.
Back in the control room, I learned that both of these lines had developed leaks at their 90-degree elbow fittings over the past year.
According to the piping schematics, the 1-inch recirculation lines had a stainless orifice plate between flanges with an orifice diameter of 0.135 inch. If the plate was severely eroded (as I then expected), it could easily flow well above 120 gpm back to the deaerator tank.
The staff engineer and I agreed to conduct a brief field trial:
We asked the boiler operator to close the isolation valves in the recirculation line on both pumps. Immediately the load on both motors changed from 82 to 46 amps.
After a few minutes of steady-state flow, we shut down pump No. 2. The amps on motor No. 1 increased from 46 to 73, about normal for a one-pump operation at 70 percent load.
Next, we switched to pump No. 2 and experienced similar results.
Finally, opening the minimum flow line on pump No. 2 increased motor load to 86 amps.
No question! The orifice plates were eroded away. Hey, these boilers were 30 years old and the orifice plates had been experiencing a 300 psig differential. Having experienced this 'plate erosion’ condition at other similar boiler plants, I offered a replacement orifice pipe design for the recirculation lines:
Use a six-inch length of 1 ¼-inch diameter cold rolled steel bar stock.
Drill a 9/64-inch hole through the center of the bar.
Tap both ends of this nozzle with 1-inch NPT threads.
The recirculation line is Schedule 80. A tight threaded joint (use Teflon tape) is effective.
This design will last indefinitely.
Commercial automatic recirculation control valves are expensive, and I prefer not to use them on low and medium pressure boilers with feed pumps at 100 hp and smaller. Generally orifice pipes are more reliable and economical. At a boiler steam load in the 25 to 40 percent range, an ARC valve is simply not accurate enough to detect a minimum pressure differential change of possibly 1 to 2 psig when the boiler drum level control valve closes for short-duration process demand changes.
Converting the ARC valve to an electronic controller (using motor amps) does not work reliably either. The amperage change is difficult to discern at low feedwater flow rates. Failure of the valve to open (and establish minimum flow) is a recipe for pump cavitation and becoming “steam bound.” If the boiler trips on “low water,” plant production can be affected immediately: Not a situation that production managementappreciates. Repeatedly tripping two tissue machines off-line because of a boiler feedwater pump mechanical issue is not good for the career path of a staff engineer.
For larger boilers with high-capacity, high-horsepower feed pumps, the ARC valve often becomes an energy conservation (cost) issue. Not continuously pumping 10 percent of the feedwater back to the deaerator, reduces a lot of motor kWh over a year; especially important in regions of the United States where electric power costs are 8 to 12 cents per kWh.
Just be very cautious about “dry-running” a $50,000, multistage, stainless-trimmed feedwater pump. T alk about having an upset boss! Been there, done that: It is not recommended!
Plant engineering later scheduled a replacement of the orifice plates in the recirculation lines with robust orifice pipes. The boiler plant returned to a single feedwater pump operation with no subsequent troubles. They went from two pumps consuming 148 hp to one unit consuming 65 hp, saving approximately $100 per day in electrical power.
If you have process water pumps that are experiencing performance problems, be sure to analyze the system technical issues. Collect operating data and compare it to the pump curve. Don’t forget the piping system schematic diagram. What may have changed recently?
A critique of the data should identify probable causes. If needed, get some advice from a pumps expert. There should be several on Google, and www.pumps-systems.com is a good resource.