Issue 5 – September 2020

What are Pressure Gauges Actually Telling Us?

This issue discusses the very important role of pressure gauges – why they must be accurate, the need to measure absolute pressure on the suction of a pump, and how to determine what is happening to a pump and how efficiently and reliably it is operating from suction and discharge gauge readings.

In our pump training courses, our trainers frequently harp on about the importance of installing pressure gauges on the pumps in your factory or plant. One trainer compares pumps without gauges to driving your motor vehicle without an instrument panel – you have no idea how fast you are going, what revs the motor is doing, or whether the engine is running hot. Pressure gauges are inexpensive, generally easy to install if there are existing pressure tappings, and are critical to understanding the operation of your pumps.

Now that all the pumps in our plant have pressure gauges, on both the suction and discharge, what do we do with this information? How will it help us operate our pumps more efficiently and reliably.

Once again, as with most topics related to pumping systems, it is not quite as simple as that. some of the questions ask are:

  • Are the gauges accurate?
  • Is the suction gauge correct for the application
  • How do we interpret the discharge pressure reading

The general state of gauges on our pumps

I have been in numerous plants where the gauges installed on pumps are in a very sorry state. Blocked pressure tappings, gauges filled with water, broken dials, out of date or missing calibration certificates are just of the obvious signs of an inaccurate reading. Blockages in the reading pipe and isolation valves not fully open exacerbate the problem. Even brand new looking gauges with clear displays does not mean they are reading accurately. Pressure gauges need to be recalibrated on a regular basis, at least once a year. This should be done by removing the gauge and sending it off for recalibration, but a pragmatic alternative is to install a calibrated gauge and take a snap shot reading while the pump is operating, and compare this with the reading off the installed gauge. You need to be able to trust the reading off the gauge.

What does the suction pressure actually mean?

The suction pressure, together with the discharge pressure, is required to calculate the total dynamic head of the pump. In most applications it will be significantly lower than the discharge pressure of the pump and can thus be ignored. As an example, a typical cooling water pump has 20kPa suction pressure and 600 kPa discharge pressure – do we really need to measure the suction pressure accurately? Even a 50% error in the suction pressure reading (10 kPa) will lead to less than 2% error in calculating the pump head. Why is it so important to ensure our suction pressure gauges are accurate (as stated above).

The most important function of the suction gauge is not to calculate the pump head, but to identify Net Positive Suction Head (NPSH) problems that could lead to the pump cavitating. We do not want our pumps cavitating as they tend to destroy their impellers and casings (as well as bearings and mechanical seals) within a very short period of time. To prevent pumps from cavitating, we need to ensure that the NPSH required by the pump (easily read off the pump curve) is lower than the NPSH available in the system. How do we find the NPSH available in the system – we read it off the suction pressure gauge. However we know that NPSH is measured in absolute pressure so the suction pressure gauge must be capable of measuring this.

To illustrate the above, assume our pump has a NPSHr of 5m (as read off the pump curve). There is an installed gauge just before the suction of the pump and we will look at two different operating conditions with two different types of gauges. Let us assume we are at sea level where the atmospheric pressure is 101 kPa.

Table 1

We know that NPSH available must be higher than NPSH required by the pump to ensure the pump does not cavitate. It is clear from the above example that going to the trouble of fitting a standard pressure gauge on the suction of the pump is a waste of time. This MUST either be a compound pressure gauge (i.e. it can go negative), or an absolute pressure gauge.

Table 1

Fig 1. Compound pressure gauge showing the onset of cavitation only starting at pressures below -40kPa gauge (-0.4 Bar). A standard pressure gauge would still be showing 0 kPa and would be useless in this application

What does the discharge pressure actually mean?

Now that we have ensured the gauges are the correct type, and recently calibrated so we are confident of their accuracy, what exactly are we measuring. We have seen how to use the suction pressure gauge to check on NPSHa and see whether the pump is cavitating. But what about the discharge pressure. For the same pump above, we measure a discharge pressure of 550 kPa. Is the pump OK? Is it running efficiently and reliably? What happens if the pressure rises to 650 kPa or drops to 400 kPa – does that mean there is a problem.

Unfortunately we are used to stand alone instrumentation in the field which tell us all we need to know, based on the single value being read. A vibration sensor reads 3.5 mm/s, and we know that anything less than 6mm/s is OK, and if it is above 10mm/s then we should immediately do something. Monitoring the temperature of bearings or the cooling water to the mechanical seals quickly identifies a problem when the temperature rises above a preset alarm level.

There is however no rule of thumb which states that that if the discharge pressure of a pump is less than a certain value, then everything is OK. Some pumps have a discharge pressure of 8000 kPa, which for a 10 stage mine dewatering pump is normal. Some pumps have discharge pressures of 80kPa, a high flow axial flow pump as an example. The problem with making use of a pump’s discharge pressure reading is that it makes no sense unless we can relate it back to the specific pump curve for that pump – and for that we not only need the discharge pressure, but also the suction pressure. What is important is the total dynamic head developed by the pump, because this will tell us how far away from its Best Efficiency Point (BEP) the pump is operating. The pump could be running far to the right of BEP, and suffering from cavitation. Or the pump could be running far left of BEP, in an area of high recirculation leading to lower efficiency, increased wear on the impeller and casing, and reduced bearing and mechanical seal life.

Table 1

Fig 2. Pump curve showing different measured operating points, and their effect on the pump efficiency and reliability

Measuring suction and discharge pressure accurately is only the first step in improving the operation of the pump. Next month’s column will discuss what to do with this information.