New Installation Methods to Monitor Vertical Water Pumps

Written by Malcolm K. Graham, Saudi Aramco and Barry Nurcombe, CEng, GE Energy

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Installation Pitfalls

While the installation of monitoring transducers on the submerged part of a vertical pump is relatively straightforward, there are several important points that need attention:

Safety-Access to the submerged part of the pump is difficult and can be dangerous. Ensure that appropriate safety measures are in place before entering the intake pit.
Turbulence-The areas surrounding the pump bowl and intake are subject to high turbulence, with significant force being generated by the swirling water. The transducer and the conduit must be securely attached to the surface, and the conduit attached to the pump body at frequent intervals. If the components that are torn off the pump by the turbulence are ingested, they will damage the impeller.
Galvanic Corrosion-Some materials may actually promote corrosion. Choose the transducer housing and the conduit materials carefully to inhibit or prevent corrosion. They must be compatible with the materials found on the pump body and in the intake pit. Sacrificial anode blocks are particularly encouraged for seawater intake pumps.
Protection of Electronics-Water and electronics do not mix well. Ensure that the transducer is properly protected against ingress of water that would lead to premature failure.
Coatings-Many pump intakes, bowls, and riser tubes are treated with special corrosion-inhibiting coatings. Considerable care must be taken to repair and seal this coating when drilling through the metal below water level.
Withdrawal Obstruction-The pump bowl may not have much clearance when it is inserted in or withdrawn from the foundation. The transducer may require a custom-engineered enclosure to keep it from interfering with the foundation.
Wake Turbulence-Any conduit passing across the intake of the bearing will cause a wake. This wake will give rise to cavitation at the impeller, increasing the risk of erosion. Keep the conduit in line with struts or webs to prevent turbulence.
Erosion-If the passage of particulate matter through the pump erodes the shaft, it will erode the probe tip, causing failure. Therefore, it is important to protect and package the probe with erosion-resistant materials.

Design and Development of a Reliable Installation Package

In 1995, GE's Bently Nevada^TM applications engineers began designing and installing special transducers for the types of vertical pumps discussed in this article. In early trials, a standard velocity sensor was attached to the pump body, then covered with layers of glass fiber and epoxy resin. While this successfully protected the transducer, it eliminated the ability to carry out maintenance.

To address this shortcoming, they then adopted technology used on submarines: namely, placing the electronics in a sturdy housing completely filled with water-repellent grease. A standard velocity transducer housing was modified by adding a pressurized grease fitting and sealing the exit with a cable seal assembly (see Figure 3). These modifications to existing hardware and the new package designs for proximity probes and restricted access installations have made proactive management of a wider range of pumps and pump applications economically feasible.

Saudi Aramco Trial Results

The custom transducer assemblies described above were provided to Saudi Aramco for a trial installation on six seawater intake cooling pumps at the Ras Tanura Refinery on the Arabian Gulf Coast. The year-long study was tasked with establishing solid evidence that the dynamic behavior of the pump is not documented by the data from transducers on the motors. A second task was to demonstrate instrumentation reliability.

Two additional transducers were mounted on the bottom of the mixed flow, 5-vane seawater pump (see Figure 4), in line with the original transducers on the motor. Signals were routed to a four-channel monitor with outputs for each sensor. Data was collected on a monthly basis with a two-channel portable spectrum and time domain analyzer.

The frequency and time domain data (see Figure 5) collected by Aramco shows a clear difference in readings taken from the sensors at the bottom of the pump and the top of the motor.

The pump data shows a high level of 1/2X component, especially in the in-line direction. This signal is highly attenuated in the motor data from the corresponding in-line direction transducer. The signal components around 340-Hz, thought to be associated with cavitation, are completely absent from the signals from the motor transducers.

The Ras Tanura trials successfully demonstrated the effectiveness of the probes and the reliability of the instrumentation in adverse conditions. The Aramco engineers recommended installation of sensors at the bottom of submerged vertical pumps. As a result, the Saudi Aramco centrifugal pump standard, 31-SAMSS-004, now includes the monitoring requirement for open pit applications and plays a pivotal role in eliminating many pump failures that came without due warning.

SEC Follows Lead of Saudi Aramco

Many customers follow standards set by others in their region. As an example, Saudi Electric Company (SEC) specified that ten vertical seawater pumps on their Ghazlan II Power Plant were to be fitted with underwater monitoring. Close cooperation was maintained with the design and project engineers of the pump manufacturer, Weir Pumps of Glasgow, to reduce the impact on the manufacturing and supply schedules. While the main cooling water pumps (CWP) were fitted with identical transducer housings as used on the Aramco pilot project, the auxiliary CWP units, which must pass through a smaller foundation ring, required custom housings. These were designed and installed as part of the erection program.

Summary

Vertical pumps are not typically considered critical when compared with main process or generating equipment. However, their failure can adversely impact production in many cases. With demands for increased profitability, the economic justification for monitoring any machine that can impact production also increases. While most would agree that the underwater section of vertical pumps should be monitored when a monitoring system can be justified, installing transducers on this section of the pump has previously been impractical due to the challenges of submerging these transducers. This article has demonstrated that capabilities now exist which make submerged transducers practical, and which provide excellent machinery protection and monitoring capabilities.

This article first appeared in ORBIT magazine. All material copyrighted GE Energy.

Malcolm K. Graham is engineering consultant for the dynamic analysis unit of the consulting services department of Saudi Aramco.

Barry Nurcombe, CEng, is sr. applications design engineer (retired) of Bently Nevada Asset Condition Monitoring, GE Energy, www.ge-energy.com/bently.

Bently Nevada and Velomitor are marks of the General Electric Company in the United States and/or other countries.

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