Pump monitoring instrumentation is often not included with installations, but proper sensors can protect a pump and help with efficient operation.
Pumps of all sizes are used throughout the process industries to transfer a wide range of fluids. Along with proper pump selection and installation to address a particular application, determining what kind of information is needed to protect the pump and help it operate more efficiently is important.
The Hydraulic Institute publication Optimizing Pumping Systems Executive Summary, states that a pump system with no means of measuring the flow, pressure or power is an inefficient pumping system. Unfortunately, instrumentation to provide information for pump protection or performance monitoring is often not specified or supplied at the time of installation. Fortunately, a modest investment in sensors can help improve the reliability and performance of expensive pumps and pump systems. In some cases, spending a few hundred or a few thousand dollars on instrumentation can protect a $50,000 pump from serious damage or help prevent even greater operation losses.
Pump protection starts by providing the proper sensors for monitoring key pump functions. If the necessary monitoring devices or provision for their installation were not included in the original pump installation, they need to be added. This does not have to be an extensive addition or upgrade.
Centrifugal pumps have a specified performance window of operating curves. Moving out of that window or moving too much within it may produce stresses that can result in damage to the pump. Installing the proper sensors, especially on more costly (in terms of the pump itself, energy usage and maintenance history) pumps, can help identify problems before they become too serious, damage the pump and impact maintenance and operation resources.
For example, installing a pressure sensor on the suction side to measure net positive suction head available (NPSHa) will help show if a pump is not running within the proper pump performance curves. Temperature or pump vibration monitoring sensors can indicate mechanical problems before they become advanced. Specifying an empty pipe detection (EPD) or low/no flow sensor can indicate when pumps have trouble.
A pump protection system that does not require running new wiring uses a self-powered WirelessHART adaptor combined with low-cost HART pressure, temperature transmitter/sensors and flow meters (see Figure 1) to help provide pump condition information. The protection system will deliver pump suction pressure, discharge pressure and temperature information to a recorder, data server or web portal on a temporary or permanent basis. The pressure differential across the pump is determined by the difference between the measured suction and discharge pressures.
To avoid cavitation conditions, the NPSHa must be greater than or equal to the net positive suction head required (NPSHr). Monitoring the suction head (pressure in terms of water column) for this condition can help identify problems that can damage the pump. A number of factors can change the NPSHr—including but not limited to increases in flow rate or changes to the head (such as pressure from fluid density or level) in a supply tank in front of the pump.
High pressure or vacuum conditions may impact some applications. Ceramic pressure sensors that can measure abrupt vacuum or pressure changes without being damaged themselves can be used in these situations. Ceramic sensor pressure transmitters can also resist a great deal of direct physical diaphragm abuse without affecting calibration even when repeatedly relocated to different pump installations.
Figure 1. Sensors can be installed to monitor pump performance by measuring suction and discharge pressure, temperature, seal-pot level and other variables.
A pump is “dead-headed” when it operates with no flow. If no fluid is entering the impeller of the pump, it will churn the same volume of fluid as it rotates. Friction will lead to increased fluid temperature which can reach the point at which the fluid flashes into vapor, disrupting cooling flow to the pump’s bearings and packing. If this condition continues, it can cause excessive pump wear and damage.
A flow switch installed at the pump inlet can alarm when the flow rate into the pump drops below a preset rate while the pump is running. For less than $400, a calorimetric flow switch can indicate a low/no flow condition with which damage can occur.
A pump is “dry running,” or in an empty pipe condition, when fluid is not sufficiently delivered to the suction side of the pump. One way to detect this condition is to specify an EPD switch. If a pump is allowed to run dry, it can cost thousands to recondition or repair. At $400, an EPD switch is cost-effective pump life insurance. Electronic tuning fork EPD switches are more reliable than float sensor EPDs in some pumping applications.
For example, an OEM skid builder had problems with floats sticking because material in the fluid coated the floats. In this application, the pump could not run dry and had to shut off when no water was available for the shaft packing. The skid builder installed tuning fork EPD switches instead of floats (see Figure 2), and false trips went to zero. No pump failures were reported because of switch malfunction, and no burnt pump packing was reported.
Figure 2. A simple $400 tuning fork level switch with empty pipe detection (EPD) keeps this pump from running dry.