By Salih Moaied P.ENG., Senior Project Manager, Aquatech Dewatering Company
There are a variety of different instrumentation devices available in the market today that can be used to virtually monitor every aspect of pumps, and pumping systems. These devices can be used both locally and remotely though a smartphone application or a web browser. One particularly useful monitoring systems is the use of pressure gauges on a centrifugal pumping system to monitor and collect data that can be used to proactivity troubleshoot system issues and prevent unnecessary failures.
Knowing the working suction and discharge pressure of a pump is one of the most critical pieces of information for any pumping system. Suction and discharge pressure readings, along with an understanding of how to read pump curves and system curves, allows a user to quickly identify the operating condition of the pumping system and make quick adjustments if necessary, to mitigate the risk of equipment damages and downtime.
Determining the Total Dynamic Head
To determine the pump operating point using the pressure gauge reading, operators will need to estimate the system total dynamic head (TDH) _ rst. TDH is the total head (energy) developed by the pump at any given set conditions and is measured is in terms of head expressed in feet or meter of liquid. Pump system designers use different equations and site conditions such as length of pipe, diameter of pipe, change in discharge, and suction elevation, to calculate the TDH. If, however the operator has access to accurate suction and discharge pressure gauges, TDH can be estimated without the need of additional system details. In this case, the following simple formulas can be used to accurately estimate the TDH and the operating point of a pumping system.
Equation 1: Head in feet = (psig x 2.31)/spgr
This equation is used to calculate the pump’s total discharge head as well as suction head when pump is under a positive feed (liquid is fed into the pump suction by gravity or other means).
Equation 2: Head in feet = (inHg x 1.13)/spgr
This equation if used to calculate the pump’s total suction head in suction lift or restricted suction application. In this case, pump must be primed in order to start moving liquid.
spgr: is the specifi c gravity of the pumped liquid
psig: is the discharge pressure gauge reading in psi
inHg: is the sucti on/vacuum gauge reading in inches of mercury
The TDH of a pump is equal to total discharge head minus the total suction head, or plus the total suction lift. If the pump’s suction is under vacuum, the water source is below the suction eye of the impeller, the suction pressure will be a negative number and the TDH formula will require that the discharge and the suction pressure readings be added together. If the water source is above the eye of the impeller, the suction pressure reading would be a positive number and the formula will require that the suction pressure be subtracted from the discharge pressure.
One important aspect to note here is that velocity head is not accounted for in the estimation of TDH using pressure gauges. For more accurate results, velocity head will need to be calculated using the dynamic pressure equation and added to the TDH calculated from the two pressure gauge readings.
Calculating Velocity Head
Velocity Head (ft, m) = V2/2g
V = fl uid velocity (m/s, ft /s)
g = accelerati on of gravity (9.81 m/s2, 32.174 ft /s2)
Consider for example, a flow velocity of 12 feet per second, with a velocity head of just 2.2 feet that increases exponentially with any increase in flow velocity. The velocity head would quickly become a factor that could easily influence the TDH results, in applications where the velocity is high, and the static and friction losses are low.
Once the TDH of the system is calculated, it needs to be imposed on the pump curve provided by the manufacturer to establish the pump’s operating point which will allow the operator to better understand the pump performance. It is recommended that the operating point falls between 80% and 110% of the best ef_ ciency point (BEP) to minimize wear and to reduce the risk of premature failure, as seen in in Figure 1. If the operating point falls far right or far left of the BEP, the operator is required to make some adjustments to the pumping system, such as increasing or reducing the pressure, to bring the operating point within the BEP range. Figure 1 also highlights the impact of moving away from the BEP and the potential of risk associated.
Reading the Pressure Gauge
There are three recommended practices that can be employed to ensure an accurate and representable pressure gauge reading:
1. Be sure to take suction and discharge pressure readings while the pump is in operation and has reached a steady state condition. Pressure readings while the pump is operational, will provide the total head of the system, including friction losses and static head. Pressure gauge reading while the pump is off provides static head only.
2. Measure pressure gauges as close as possible to the discharge and suction _ ange of the pump. Total head is proportional to the difference between these two measurements. Pressure adjustment will be required if the pressure gauge is placed higher or lower than the pump eye of impeller (centre line).
3. Pressure measurement must be taking by a calibrated gauge in a pounds per square inch gauge (psig) or inches of Mercury (inMH).
An alternative way to estimate pump’s operating point is to use a flowmeter to determine the flow rate of the system and in turn, estimate the operating point along the pump performance curve. If the system is not equipped with a flowmeter, a clamp-on portable flowmeter can be used to measure flow rate in most cases. Typically, these flowmeters require smooth flow free of particulates or vapor and the pipe is flowing full unless the meter is equipped with a depth senor for use in partially full pipes.
The use of pressure gauges by an operator on a pumping system is like the use of Stethoscope by a doctor to listen for heartrate. Usually, one of the first steps a doctor preforms is to take the patient’s heart rate and decide if and what the next steps are. In pumping systems, taking an accurate suction and discharge gauge readings should be one of the first steps in diagnostics. Taking the readings with the pump in operations for TDH purpose, and when the pump is off for static head purpose, can add additional value to the operator. The results will give the operator a good idea of the pump and system conditions and whether additional tests are require.
As seen in Figure 1, if the calculated duty point falls to the right of the curve and outside the recommended range, increasing discharge pressure by closing a valve or other means can help move the operating point into the recommended range. On the other hand, if the operating point falls to the left the curve, reducing the discharge pressure by opening a valve can move the duty point to the recommended range. Sometimes, all it takes to prolong the life of a pump, and/or to prevent system damages, is simply adjusting a discharge valve i.e. closing a valve to increase pressure, or opening a valve to reduce pressure.