Measure shaft speed to determine the pump shaft revolutions per minute (rpm), which can have a great effect on the pump's head. Most tachometers, even inexpensive ones, are accurate 1 rpm ," plenty accurate for most pump field tests. You can check speed using contact or non-contact tachometers. Take care to follow all required safety guidelines when working with an exposed shaft on rotating equipment.
Voltage accuracy generally is not very important unless the voltage is found to be significantly different (generally 10 percent) from the motor nameplate voltage. Any moderately accurate multimeter will suffice. If the voltage is outside the 10 percent, contact the motor manufacturer. Take voltage measurements as close as possible to the motor junction box, like amperage measurements.
Safety is the first consideration in testing, so follow all U.S. Occupational Safety and Health Administration (OSHA) and corporate guidelines or safety procedures during the test. Collect the data after the pump and the process temperatures stabilize. If the process has multiple operating conditions such as on-shift and off-shift in a one-shift facility, check each condition point. It might be possible that the pump runs right at the BEP during a nine-hour day shift, but drops below the minimum flow rate when the facility goes off-shift at night.
Still more data
Motor performance data also are important. Most electric motor manufacturers provide data for common models on their Web sites. Data for specialized motors often require a call to the local sales office or an applications-assistance line. These data generally are provided in tabular form. I find it convenient to plot these data in a spreadsheet.
If you do not have a pump performance curve (See Fig. 2), you'll need to contact the manufacturer. Make sure the curve displays your pump's impeller trim diameter and represents the speed at which you are operating the pump.
Figure 2. Pump Performance Curve
A pump perfomance curve should include the impeller's trim diameter and the pump's operating speed.
To improve the odds of getting the correct performance curve, record as much data as possible from the pump data plate. Make sure you get the model and serial number and the impeller diameter. Many pumps are shipped from the manufacturer with a full-trim impeller and then trimmed to the required diameter by a distributor.
You also will need to know the speed at which your pump is operating. This is simple if you have a fixed-speed unit, but becomes more cumbersome if the pump uses some form of speed control such as a variable- frequency drive (VFD). If speed control is in place, use tachometer readings to provide the manufacturer with a range of operation.
If a curve is not available for the correct operating speed or if you cannot obtain a curve for your trim diameter, you can use affinity laws to approximate pump performance. Using data from an existing performance curve at a different speed or diameter, you can calculate the data for the curve you need. The formulas are shown in Fig. 3. They can be used to change diameter, speed or both. The results of these calculations are approximations of the actual values. The affinity laws assume that efficiency is constant for small changes in speed or diameter.
A free downloadable Microsoft Excel spreadsheet that does the conversions for you can be accessed at www.mechtronixeng.com. Many existing software programs also will handle the affinity law calculations. Many manufacturers provide data in an electronic format, either in a proprietary program or commercially available software. One example is Pump-Flo from Engineered Software (www.pump-flo.com).
These programs often allow the user to adjust the performance curve for changes in impeller diameter, speed, viscosity, specific gravity, etc. The user then can plot a custom performance curve for the application.
The fluid supplier probably has the best data for your fluid. You also will need to get the fluid's vapor pressure, viscosity and specific gravity at the process temperature.
After you collect the data, you need to process them and plot them on the performance curve.
My company was asked to help resolve a pumping problem at a chemical facility. The pump has a 3-in. suction x 2-in. discharge x 13-in. impeller. It has a horizontal end suction design and is powered by a 25-horsepower (hp) motor (4 pole, 60 Hz/1,800 rpm). It pumps a 68 Degrees F saturated formic acid solution.
The pump is sized (the design operating point) to provide 300 gallons per minute (gpm) at 140 ft H2O. The plant is at sea level, and the local atmospheric pressure is 33.94 ft H2O (29.92 in. mercury [Hg]).
The pump is not providing the required flow and pressure. Maintenance installs an ultrasonic flowmeter, pressure gauges, an ammeter, a tachometer and a voltmeter. Personnel record the following data:
Flow rate = 240 gpm
Suction pressure = 23.5 in. of mercury or 26.58 ft
Discharge pressure = 26 pounds per square inch gauge (psig)
Tachometer = 1,787 rpm
Ammeter = 17.5 A
Voltage = 230 volts AC (VAC)
Based on the flow rate and the suction and discharge pipe sizes, the velocity heads are found to be:
hvs = 1.69 ft and hvd = 8.18 ft
From references, the fluid's properties at the pumping temperature are:
pvp = 0.65 psia at the pumping temperature
s.g. = 1.221
viscosity = 1.45 centipoise (cp) (viscosity change is
negligible ," no correction needed)
First, calculate the NPSHA:
hgs = [23.5 in. Hg x 1.131(ft H2O/in. Hg)] = 26.58 ft
hs = -26.58 + 1.69 + 0.5 = -24.39 ft
hvp = (0.65 x 2.31)/1.221 = 1.23 ft