How to determine the abnormality and cause.
by Dominik Fry
May 24, 2019

System troubleshooting can be a difficult process for the most experienced professionals. Securing the necessary data to support your conclusions can be even more challenging. Before getting started, you will need to know how the system was designed, how it is supposed to operate, and where on the curve your pumps are operating, among other factors. In this article, I will walk you through a system that is operating correctly and two problem scenarios that are often encountered with this system. I will also explain some common pitfalls­—and tips on how to avoid them.

When it comes to gathering data on a system, building services are among the hardest. Most of the piping system may be hidden behind equipment or built into the physical structure, such as the walls and ceiling, and instrumentation is often few and far between.

For example, one engineer recently visited a mechanical room where more than half of the pressure gauges on the pumps were broken or outside of their rated range. In this type of situation, it is important to never speculate on how well they were calibrated, and when comparing these systems, you may have to get a little creative. Do not get discouraged, and do not attempt to take exact measurements in this instance (but you can still receive general data).

For instance, if the pressure gauge only goes to 80 pounds per square inch (psi), but the needle is slightly past 80, do not attempt to calculate pressure because you have passed the instrument’s range and any readings will be unreliable. What you will know, however, is that there is a fault somewhere in the system causing it to operate outside of its design parameters.

As all components of a piping system are interconnected, a problem with one component will impact the hydraulic performance of the entire system. Because of this, we must be able to understand the symptoms, and then focus on the root cause of the problem.

To start, we need to know what is considered normal. If the system has been operational for some time, then the systems operator should have a decent understanding of what normal is for their system. However, it is always helpful, whether you have worked with the pumping system before or are new, to build a flow model to give you a better understanding of what is going on and what normal should be. For this example, we will say that the pressure, flow, level and valve position readings in Image 1 are the normal steady-state operating conditions for this system.

normal operationImage 1. System under normal operation (Images courtesy of the author)

Normal Operation

  • The supply tank is open to atmosphere and its level is maintained at 10 feet.
  • The supply pump has a suction pressure of 15 pounds per square inch gauge (psig) and a discharge pressure of 80.5 psig.
  • The heat exchanger has an inlet pressure of 67 psig and an outlet pressure of 57 psig, for a total differential pressure of 10 pounds per square inch differential (psid) at 1,000 gallons per minute (gpm).
  • The flow control valve is 85 percent open with 1,000 gpm flowing through it.
  • The product tank is pressurized to 25 psig and has a level of 15 feet.

After being placed into operation, the system began to experience a variety of issues. One operational note is that the flow rate from the product tank is reduced to maintain the tank level, which limits the flow rate for the entire system. While this is an issue, it is not the main system problem.

system with initial issueImage 2. System with initial issue.

Take a minute, and compare Image 1 with Image 2. See if you can identify what is causing the system issue in Image 2.

A good first step is to look at how everything compares to the normal system operation. Here is a quick summary of how the operation differs.

  • supply tank level: OK
  • pump suction pressure: OK
  • pump discharge pressure: higher than normal
  • pump dP: higher than normal
  • HX inlet pressure: higher than normal
  • HX outlet pressure: lower than normal
  • HX dP: higher than normal
  • FCV: open farther than normal
  • system flow rate: less than normal
  • product tank level and pressure: OK

What is abnormal, and what could cause it? It would be understandable to look at the pump first, as the discharge pressure and the differential pressure are higher. This indicates the pump is producing more head, but this is expected because it is running farther back on the pump curve at the lower flow rate. The heat exchanger inlet pressure is also slightly higher, but this is due to higher pump discharge pressure.

The total pressure drop across the heat exchanger is 18 psi at 940 gpm. Under normal operating conditions, it should be 10 psi—an even higher flow rate. This would indicate that the heat exchanger has been plugged up, and thanks to this flow model we have solved the mystery of this system’s main problem.

System with expansion issueImage 3. System with expansion issue.

Let’s take a look at another system example. The system in Image 3 was built with a system expansion in mind, but they ran into issues during implementation. A trial run was done to determine the maximum flow rate possible, and even with the flow control valve at 100 percent open, the maximum flow rate was just over 1,000 gpm. Compare the system in Image 3 with the system in Image 1, and see if you can determine the root cause of this system’s problem.

Look again at how these devices compare to normal.

  • supply tank level: OK
  • pump suction pressure: OK
  • pump discharge pressure: lower than normal
  • pump dP: lower than normal
  • HX inlet pressure: lower than normal
  • HX outlet pressure: lower than normal
  • HX dP: OK
  • FCV: open farther than normal
  • system flow rate: normal
  • product tank level and pressure: OK

Were you able to determine the abnormality and the cause? The flow control valve is open farther to maintain the same flow rate, and this could be due to the lower inlet pressure. The system pressures downstream of the pump are lower than normal, and so is the differential pressure across the pump. This indicates it is producing less head for the same flow rate of 1,000 gpm, so that it is operating below its pump curve. This shows that there is a pump problem, which could be caused by a damaged impeller, worn wear rings or a clearance issue.

In any system, there are as many potential problems as there are components, and as you increase the number of branches, loops and/or multiple malfunctions, it can become more challenging to discover the root cause of your problem.

Having a good model or digital twin of your system can let you make changes and see how the system interacts with itself, regardless of complexity. I hope these examples have given you insight into the troubleshooting process and given you new tools for your troubleshooting toolbox.

To read more Pump System Improvement columns, click here.