Monitoring Pump Performance

Among the alternatives to emissions-prone mechanical seals used in traditional process pumps are canned-motor pumps (CMPs), such as this one manufactured by Teikoku USA Inc. (Photo: Teikoku USA Inc.)

“Measurement is the first step that leads to control and eventually to improvement.”—H. James Harrington

Tightening standards for reduced emissions is driving innovation, invention and increased safety within the fuels sector and, more broadly, manufacturing. The Greenhouse Gas Protocol created by the World Resources Institute and the World Business Council for Sustainable Development defines scope 1, 2, and 3 emissions as:

Direct from equipment owned by the producer/ manufacturer.
Indirect from energy usage.
Indirect emissions from the full value chain.

These definitions are the latest effort to define greenhouse-gas (GHG) reduction strategies. The WRI’s work alongside U.S. EPA’s Clean Air Act creates the push for startups and legacy manufacturers to shift away from decades-old, noncircular processes as well as emissions-prone mechanical seals utilized in traditional process pumps. Among the alternatives to mechanical seals are canned-motor pumps (CMPs). CMPs resolve scope 1 and scope 3 emissions in pump services and provide true secondary containment.

Further, CMPs can be instrumented in several unique ways to enhance safety and be an operating resource. Performance monitoring using data analytics helps owners, engineers and operators to parse large data sets to improve system performance. Figure 1 below provides a visual breakdown of where these sensors are applied as referenced in API 685, Sealless Centrifugal Pumps for Petroleum, Petrochemical, and Gas Industry Process Service.

Figure 1: Teikoku diagram—API instrumentation. (Image: Teikoku USA Inc.)

The table below describes key instrumentation and devices that may be used to collect essential information about the condition of a CMP.

Information from API 685 2nd Ed. Annex E. Table.1 Instrumentation and Protective Systems.

Figure 2: Radial position monitor for bearing wear output. (Image: Teikoku USA Inc.)

Of the API 685 recommended instrumentation, the two most utilized by CMP users and specifying engineers are the shaft-position sensors (ZE) and the power monitor/ power sensor (JE). The shaft-position sensor can be provided with either a simple radial or a combination radial and axial position sensing device. All CMP designs provide a radial shaft-position sensor as standard. In general, these devices use the Hall Effect, whereby the intensity of a magnet’s field is measured by a pair of coils. The permanent magnet is imbedded in the rotor and the coils are located in the hermetically sealed stator windings. The change in field strength measured by the coils provides an inference of the rotor’s change in radial position relative to the fixed-position stator—thus, inferring any physical wear to the bearings.

In addition to radial position sensing, a linear variable differential transformer can be installed on the rear of the motor to provide axial shaft position, typically used in cases where an axial thrust bearing is installed. The axial sensor coil has one primary coil and two secondary coils. A rod is affixed to the rear side of the rotor and is adjusted so that it travels along the axis of the coils and an AC voltage is supplied to the primary coil. A voltage is induced in the secondary coils based on the change in axial position, allowing the device to compare the voltages from the original condition against the worn condition to provide a corresponding output. Both sensors’ signals can be converted to a 4-20 milliamp or 1-5VDC signal.

Figure 3: Axial position monitor concept and meter output. (Image: Teikoku USA Inc.)

While some operations possess resources to use transmitters on the suction and discharge piping of process machinery, for many the budget is limited. In such cases, a power monitor (JE) offers the most consolidated way to infer a pump’s operating condition. It can also detect:

If fluid properties have changed.
If the rotor is locked.
Dry running condition.

In conjunction with shaft-position sensing, users can detect and track process upsets and create predictive reliability and maintenance analysis of these pumps. Lastly, vibration monitoring is an additional method to further refine the analytics of a pump. In cases where users have developed years of vibration data, vibration monitoring could potentially replace both power monitoring and shaft-position sensing as a predictive tool.

The remaining portion of API 685’s recommended monitoring offers single-variable monitoring outputs to indicate specific elements of CMP performance worthy of consideration by users in gauging pump functionality. These include:

TE2—Circulation flow path temperature.
PE—Secondary pressure casing.
TE1—Thermal cutout device.

Particularly for volatile fluids, monitoring of the circulation flow path temperature (TE2) should take place at the highest temperature point in the flow circuit. In simple internal circulation (API Plan 1-S), this is at the rear bearing housing to ensure that the circulating pumped liquid remains in the liquid state throughout the entire circulation path within the motor’s primary containment. For both volatile and nonvolatile fluids, such temperature measurements can also be used to find any obstruction in the flow path. The thermal cutout (TCO) switch (TE1) is wired in conjunction with the motor starter or as a resistive thermal diode (RTD) embedded in the motor winding. Both serve to detect when motor-winding temperature poses a risk to the winding’s insulation system and either automatically cuts power to the motor (TCO switch) or signals to operators that the pump needs to be shut down (RTD).

Use of any or all of these instruments would be appropriate to provide resolution to a site’s layers-of-protection analysis (LOPA). If a user needed to select just one monitoring device, however, a leak-detection instrument for the interior of the secondary pressure casing should be considered (PE). This is commonly achieved by using a pressure transmitter that can detect fractional changes in pressure—less than 1 pound per square inch (PSI) or millibars. If mechanical breach of the secondary containment occurs, this transmitter can work with the plant-control system to shut down the pump.

By monitoring all or part of these variables, a canned-motor pump’s superior mechanical design provides end users of any chemical process a significantly reduced risk profile.