Bright Ideas – Troubleshooting 101
By Mike MillerEngineering HPAC General Hydronics Plumbing commercial pump curves pumps troubleshooting
From home maintenance to commercial mechanical room operations, problem solving always requires a plan.
We have all had opportunities to do troubleshooting, and it seems as though I have been in the troubleshooting business the majority of my professional career.
I am often asked, “What is troubleshooting? Where do I start? And what do I do?”
In its purist form, troubleshooting is the process we go through to solve a problem.
As for where to start? My response is always: start at the beginning.
And when it comes to: “What do I do?” And the answer is: “Develop a plan.”
I’m going to share a simple example of my thought process when it came to solving a small problem at home, and then I will provide you with a real commercial pump troubleshooting situation that I was asked to solve not too long ago.
So, earlier this year I left the office on my way home. The commute is little over an hour. When I first got on the road, I called my wife to give her an estimate as to my arrival time. She informed me that one of the lamps in the living room was no longer working.
She also said that she replaced the light bulb with a new one that she knew was working because she tested it in another lamp.
The new bulb did not illuminate. I told her that I had about an hour or so of driving and would think about it on my way home.
Following my own advice, I started at the beginning and started to list the things that I knew and those that I did not know.
- I knew there was electric power available because my wife tested a bulb in another lamp.
- I also knew there were two lamps in the living room. One lamp was activated by a wall switch and the other lamp was plugged into a live electrical outlet.
- I know that the lamps are about 26 years old which is the same age as the circuit breaker, the wiring and the receptacle.
- What I did not know was which lamp was not working, the wall switched lamp or the live receptacle lamp.
Time to do some interviewing.
I called my wife and asked several questions. Her answers revealed the following:
- the lamp in question is connected to the live receptacle, and that the lamp was working fine the day before.
I could now focus my attention on the lamp connected to the live receptacle which allowed me to develop a plan.
I had about 45 minutes of commuting left, plenty of time to develop a comprehensive plan.
I now knew that the problem was either with the house electrical system or the lamp. The house electrical system consisted of the circuit breaker, the house wiring and the receptacle. The lamp consisted of the electrical cord, the lamp switch and perhaps the light bulb.
To eliminate the house electrical system, my plan upon arriving home was to turn off and on the electrical breaker at the electrical panel.
The next step was to inspect the lamp switch, the lamp wiring and the receptacle. I have a digital volt/ohm meter, which would make testing the receptacle, the lamp wiring and the lamp switch a breeze.
By this time I was almost home, and I felt confident that I had worked out a comprehensive plan to diagnose and solve the problem.
Upon arrival I asked my wife to put the wall switched lamp on so that I could check the circuit breaker. I went into the basement, walked over to the electrical panel and identified the breaker for the living room. Fortunately for me the breakers were well labeled, and I immediately found the appropriate switch.
On and off went the breaker, and on and off went the wall switched lamp. The breaker was not the problem.
I grabbed my volt/ohm meter and went upstairs to the living room. Before proceeding any further, I decided to replace the lamp bulb just in case. I removed the illuminated bulb from the wall switched operating lamp and installed it in the offending lamp. Just as my wife indicated earlier, the bulb did not light up.
The next step was to use my volt/ohm meter and test the receptacle that the lamp cord was plugged into. In order to access the receptacle, I had to move the sofa away from the wall because the receptacle was hidden by the sofa.
As soon as I moved the sofa to reveal the receptacle, I had my answer.
At this point you might have guessed that the lamp cord was not plugged into the wall. But you would be wrong.
You see, several years earlier, I purchased a digital timer which can be programmed to activate and deactivate a device such as a lamp based on the time of day.
The digital timer has an override switch (or on switch) which allows the digital timer’s outlet to be on continuously. I removed the digital timer and plugged the lamp directly into the wall receptacle and the lamp bulb immediately illuminated.
The digital timer failed and no longer provided power to the lamp.
My troubleshooting steps were correct, and I followed my plan.
However, I could have determined the problem earlier if I had been more comprehensive during the interview process.
I should have asked my wife to look behind the sofa to see if the lamp was plugged in. That simple request would have revealed another possible course of action and I would have modified my plan.
As promised, let me walk you through a real-life pump troubleshooting problem that I was asked to solve.
I received a call from one of our customers regarding what was perceived to be a pump problem. I asked the customer to provide a brief verbal description of the issue. He revealed the following:
- The pumps were installed about 10 years ago and have been in service since.
- The customer is concerned that the flow is less than the original design (approximately 850 GPM) and that the flow may have been less than design for the entire 10 years.
- There are five pumps installed, four operating in parallel one standby.
- The original design flow was 1,200 GPM and 25 FT for each pump.
- The pumps serve a condenser water system with an open cell cooling tower.
- The cooling towers use an indoor sump.
- The flow was estimated by measuring the pressure drop across the chiller condenser water barrel with one pump running.
- The pressure differential was measured across the pump at approximately 25 FT.
I asked for some additional information, including:
- One line diagram of the piping system.
- Photos of the piping and pumping system.
- Water level in the sump relative to the centerline of the pump.
- Photos of the pump and motor name plates.
- Pump speed in RPM.
- Water temperature.
- Suction pressure with the pumps off measured at pump suction flange.
- Suction pressure with the one pump running at pump suction flange.
- Discharge pressure with the one pump running at pump discharge flange.
- Voltage and amperage at the VFD input with motor operating at design load.
After my initial interview on the phone, while waiting for him to send along his information, I collected some documentation including pump data, motor data and drive data.
Now it was time for me to develop a plan.
I decided to create a four-step plan. The first was to review the published data along with the drawings, and the second was to review the information provided by the customer. I needed to evaluate the data before moving on to steps three and four.
The line diagram along with the photographs indicated that the suction header was at the same elevation as the pump suction connection. In other words, the centerline of the suction header was at the same elevation as the centerline of the pump suction.
The photos also revealed that the discharge gage was reading 11 PSIG (approximately 25 FT) and the suction gage was reading 0 PSIG (0 FT) for a differential pressure of 25 FT at 60 Hz.
The elevation of the water level in the sump was 4 FT above the centre line of the pump. At this point I had a hunch that the suction gauge may not be giving us the actual suction pressure. I contacted the customer and asked for a short video of the suction gauge while the pump was running. The video revealed that gauge dial was resting on the gauge pin at 0 PSIG with no movement.
Time to do some calculations.
The suction header was 20-in. in diameter and approximately 30 feet long and was connected directly to the cooling tower indoor sump. Calculations proved that with one pump operating, the pressure drop was negligible in the suction header. Therefore, the pressure at the pump suction pipe where it connected to the header should be + 4 FT.
Again, back to the drawings and the photos to determine what was between the pipe connection at the header and the pump suction connection. This revealed a butterfly isolation valve and a basket strainer. I now had enough information to complete my plan.
I asked the customer to replace the conventional suction gauge with a compound gauge. A compound gauge can read pressure values both above and below 0 PSIG.
I also asked the customer to take suction pressure readings with the basket strainer screen both in and out.
Here is what he recorded:
- With all the pumps off the suction pressure at the pump flange read + 4 FT (basket strainer screen in)
- With one pump running at full speed (1760 RPM) the suction pressure at the pump flange read – 25 Inches of Hg (approximately – 12 PSIG with basket strainer screen in).
- With one pump running at full speed (1760 RPM) the suction pressure at the pump flange read + 3.5 FT (basket strainer screen out).
These new readings proved that the actual pump pressure differential was 25 FT – (-12 FT) or 37 FT.
At this pressure differential the pump curve indicated the flow to be approximately 850 GPM. The problem was not the pump but the unanticipated pressure drop of the basket strainer (with screen in).
I recommended that the customer investigate replacing and/or relocating the basket strainer—problem solved.
Admittedly not all troubleshooting problems will be this easy to solve, but the principals are the same.
- First, identify the problem.
- Second, collect data including manufacturers data, drawings, diagrams, field measurements, photos and videos (you can never have too many photos and videos).
- Third, do some calculations (if appropriate), and finally develop a plan.
Have fun on your next troubleshooting adventure and drop me a line to let me know how you are making out. <>