Pumps of all sizes are used throughout the process industries to transfer a range of fluids. Along with proper pump selection and installation to address a
Causes - Pump Damage
Checking Seal Pots
Measuring Pump Efficiency
particular application, it is important to determine what kind of information is needed to protect the pump and help it operate more efficiently.
The Hydraulic Institute publication, “Optimizing Pumping Systems — Executive Summary,” says that a pump system with no means of measuring flow, pressure or power is an inefficient pumping system. Unfortunately, the instrumentation to measure and report pump conditions — both to protect the pumps and to allow performance monitoring — often is not specified at the time pumps are installed. Fortunately, a modest investment in sensors can help improve pump and pumping system reliability and performance. In some cases, spending a few hundred or a few thousand dollars on instrumentation can protect a $50,000 pump from serious damage — and head off 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 pumps (in terms of the pump itself, energy usage or maintenance history), can help identify problems before they become 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, and 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 can help mitigate the cost of adding instrumentation after a pump is installed. For instance, one system on the market uses a self-powered WirelessHart adapter along with Hart pressure, temperature transmitter/sensors and flowmeters (figure 1). It can display pump condition information and deliver pump suction pressure, discharge pressure and temperature information to a recorder, data server or web portal.
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 conditions that can damage the pump. A number of factors can change the NPSHr, including increases in flow rate or changes to the head (that is, pressure from fluid density or level) in a supply tank in front of the pump.
High pressure or vacuum conditions can affect some applications. Ceramic pressure sensors, which can measure abrupt vacuum or pressure changes without themselves being damaged, can be used in these situations. Ceramic sensor pressure transmitters can resist a great deal of direct physical diaphragm abuse without affecting calibration, even when frequently relocated to different pump installations.
Another common cause of pump damage or failure is deadheading, which occurs when the pump is operated with no flow. If fluid is not entering the pump’s impeller, the pump will churn the existing volume of fluid as it rotates. Friction will lead to increased fluid temperature, which can reach the point where the fluid flashes into vapor, disrupting cooling flow to the pump’s bearings and packing. If sustained, this 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. A calorimetric flow switch can indicate a low/no flow condition where damage can occur.
Dry running — or operating a pump with an empty pipe condition, where fluid is not delivered sufficiently to the suction side of the pump — also can damage an installed pump. If a pump is allowed to run dry, it can cost thousands of dollars to recondition or repair.
One way to detect dry running is to specify an empty pipe detection (EPD) switch. Electronic 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 it had to be shut off when there was no water for the shaft packing. The skid builder installed EPD switches instead of floats (figure 2), and false trips went to zero. Pump failures were not reported due to switch malfunction, and burnt-up pump packing also was not reported.
EPD switches also are available within some flowmeters. Most pump systems can benefit from monitoring via a flowmeter, and specifying one with an EPD switch can add an extra level of protection.
If a flowmeter, flow switch or EPD switch is not installed in an application, a temperature sensor sometimes can be used to sense some of the problems that can occur from low or no flow. The temperature sensor must react quickly before damage can occur. A pump temperature sensor (figure 3) using thin-film technology reacts more quickly than standard temperature sensors.
Pumping systems involving toxic, hazardous or corrosive fluids cannot tolerate leaks into the environment. In such cases, a pump may have a double or tandem seal that uses a compatible liquid injected into a seal chamber to serve as a barrier fluid. If any hazardous product leaks across the inner (primary) seal, it enters the seal chamber and mixes with the barrier fluid. Improper control and maintenance of the seal environment can result in total seal failure, along with unsafe leakage of undesirable products into the environment.
Supply tanks — called seal pots — serve as a reservoir of clean, pressurized barrier fluid for double- or tandem-seal assemblies (figure 4). Fluid loss would result in catastrophic failure of the seal and dangerous conditions. Therefore, operators must know if the fluid level drops too low, indicating a fluid loss problem. These seal pots generally have either a high-level or low-level liquid switch, or both, depending on the application.
The low-level switch indicates loss of barrier fluid; the high-level switch indicates hazardous product leakage across the primary seal and into the seal chamber (thereby adding fluid volume and raising the seal pot level). Outputs from these switches connect to alarms or the plant control system, calling for immediate corrective action. All seal pots use a pressure sensor, with some systems requiring a pressure switch to activate an alarm, or to shut down the pump for added safety.
Proper piping, valving and pump design can avoid some conditions where pump damage or failure can accelerate. For example, an automatic flow-control device or a recirculation line between the pump’s discharge and source lines can ensure enough flow though the pump to prevent overheating or damage.
But, if operation outside the pump specification cannot be ensured by piping design, then instrument-based monitoring should be considered. In the real world, precise pump sizing and piping design does not always take place up front and, even when it does, changing operating conditions can render initial calculations invalid.
Pumps are vital, expensive systems that consume a great deal of energy and are expensive to rebuild or replace. The addition of a few sensors to a pump system can help make pumps run more efficiently and help processors discover problems before they get too serious.
Increasing the efficiency of pumps can result in savings. Pumps are one of the highest energy consumers within many process operations. A study by the Department of Energy confirms that pumps consume about 50 percent of the energy used in a plant. Therefore, the potential savings by increasing pump efficiency can be considerable. To find out, a plant should do a pump efficiency study ― even if the pumps were carefully selected initially.
Although a pump may have been sized for optimum performance for a given pumping system, a pump may be subjected to different operating conditions than expected. From the original pumping system design, process adjustments over time can shift the fluid densities, flow rates, pressures, temperatures and viscosities the pump has to handle.
Many pump performance-monitoring approaches use sensors for suction pressure, discharge pressure, flow rate, pump speed and power use. From these variables and other details on the pumping system, pump efficiency and condition can be determined.
Pump efficiency is defined as the pump’s fluid power divided by the input shaft power. It is influenced by hydraulic effects, mechanical losses and internal leakage.
For pumping system performance monitoring, one can specify that some of the same sensors used in a pump-protection system also provide information in a pump performance-monitoring system. For instance, suction and discharge pressure sensor information, flowmeter information and pump power and rotational speed information can help designers and operators determine the pump’s performance relative to its best efficiency point (BEP). In some cases, it may be determined that the current pump system cannot meet current operating requirements. Alternately, performance monitoring may show that a pump is incorrectly sized and needs to be reworked or replaced to achieve energy and maintenance cost improvements.
Accurate flow rate data provides key indication of pump efficiency. Suitable flowmeters already inline with the pump often can be used to provide pump flow performance information to compare with pump power and speed information over time. Ideally, of course, the role a process flowmeter takes in lifecycle pump performance monitoring is best specified during the design of the process and the pumping system.
If a flowmeter is not already suitably positioned to provide flow rate information at the pump, portable ultrasonic flowmeters often can be mounted temporarily on existing piping near the pump. These clamp-on ultrasonic flowmeters are convenient, but care needs to be taken to follow installation requirements to ensure getting usable flow rate data that fits the performance-monitoring needs. It is critical that the installed flowmeter have good calibration history to ensure data is accurate enough to represent the true flow rate though the pump.
As an alternative, coriolis flowmeters can be installed with little regard for upstream and downstream flow profiles even on large pipes. Modern coriolis flowmeters can provide traceable flow accuracies of ±0.05 percent.
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