December 2016 — Pumps are designed to accomplish two tasks: overcome friction and move fluids by changing the pressure of a fluid or gas. Friction losses are head losses in the piping system. The magnitude of these losses is determined by the liquid being pumped, pipe size, type of pipe, flow rate, and the number of bends, valves, and other restrictions in the piping system. Fluids always flow from a higher pressure to a lower pressure.
Verifying Proper Pump Operation
Check all pumping systems periodically to make sure they are operating properly. A routine schedule should be established to check pressure gauges, valves, bearings, mounting bolts, seals, and temperatures.
Pressure gauges on a pumping system should be located on both the suction and discharge side of the system. The difference in the readings of the gauges provides the actual pump head. Using the gauge readings information from the gauges and the impeller size, calculate the flow rate from the pump performance curve to ensure that the pump is operating at maximum efficiency.
Regularly test all the valves on the pumping system to make sure they have not seized or become stuck. Although most pumps include balancing valves, isolation valves, and check valves, some systems may have combination valves that encompass each of the individual valve duties. Balancing valves restrict the flow of the liquid at the proper rate until the pressure drop reaches the specified level. Isolation valves are used to separate a component to be serviced from the rest of the system. Check valves keep the liquid from flowing back into a pump that has been shut down.
Periodically check pump shaft seals for leakage. Verify that the pump packing is dripping at the proper rate.
Make sure the pump and the motor bearings are properly lubricated and that the bearings and seals are not running at excessively high temperatures. Most bearings can operate to at least 160°F without problems.
Pump Shutdown
Before stopping a pump, verify that the task for which the pump is being used can be interrupted. Under normal circumstances, shutting down a pump requires only turning off the starter. After the pump has been turned off, isolate it from the rest of the piping system by closing the valves on both the suction and discharge sides of the pump.
Pump Maintenance
Pumps can run for years with minimal attention. It is important to note, however, that this does not mean that pumps will run forever without any attention. Only a few parts on most pumps need maintenance. Centrifugal pumps need maintenance performed on the bearings, seals, and drive systems. Valves will eventually wear. Establish a maintenance schedule for the valves based on the number of pump running hours. Manufacturers usually provide maintenance guidelines that are designed to allow maximum equipment life.
By performing routine inspections of the equipment, operators can usually identify and solve problems before they become major sources of trouble. Because there are so many different types of pumps, the guidelines outlined here will not cover every situation, but they do outline common maintenance for centrifugal pumps. Be sure to review the manufacturer’s recommendations for any unique features.
Record the following items in a daily log:
- Suction and discharge pressure
- Suction and discharge temperature
- Makeup water in GPM
- Pressure drop across the strainer
Inspection
Regularly inspecting pumps for leaks, lubrication, drive system wear, noise, tight electrical connections, proper voltage, and amperage and vibration is crucial for pump longevity. Correct any problems as soon as they are discovered.
Leaks
Damaging leaks will most likely occur around the stuffing box that contains the packing or mechanical seal. If the pump has packing in the stuffing box, there should be a slight amount of liquid dripping through the packing. This can be as much as 40 to 60 drops per minute. The purpose of the liquid trickling out through the packing is to lubricate and cool the packing so it does not overheat. Some packings are designed to be externally lubricated by means of a lantern ring that allows a lubricant to be inserted into the packing at a higher pressure than the internal pressure on the packing. The lubricating liquid can be water, oil, or some other compatible lubricant.
If the pump has a mechanical seal, there should be no liquid coming from the seal. If there is any leakage, investigate the cause and fix the problem right away.
Mechanical Seals
A mechanical seal is a method of sealing between stationary and rotating parts and is used on many pumps to create a seal between the impeller shaft and the casing. The mechanical seal is used on centrifugal pumps to prevent liquid or gas from leaking along the impeller shaft and to prevent wear on the shaft that may occur if a packing seal is used.
A mechanical seal has the following advantages over a conventional packing seal:
- No leakage of the liquid being pumped
- Elimination of shaft or sleeve wear
- Reduced friction and power losses
- Reduced maintenance
A mechanical seal consists of three seals.
- A static seal between the stationary seal and the pump housing. This is normally an “O” ring seal.
- A static seal between the rotating seal and the pump shaft. This is normally an “O” ring seal.
- A dynamic seal between the rotating seal and the stationary seal normally consists of two flat sealing faces, one located on the rotating seal part and the other on the stationary seal part. The dynamic seal is located at the mating faces of the stationary and the rotating seal. Liquid pressure from the pumped liquid forces the faces of the rotating and stationary seal parts together thus forming the dynamic seal. A spring normally holds the sealing surfaces together when the pump is shut down and also allows for wear at the sealing surfaces.
The mechanical seal principle is the same for all brands, even though the form, shape, style, and design will vary depending on the application and the manufacturer.
If a mechanical seal fails it is usually due to one of the following reasons:
- Lack of lubrication: To operate properly, the mating faces of the mechanical seal must have a thin film of liquid between them to reduce the friction to the minimum. If there is no film, the seal will overheat and fail. It is extremely important that a mechanical seal never be run dry.
- Misalignment: If a mechanical seal is not installed properly and the mating faces are not parallel to each other, the mating faces will wear on each other unevenly. This causes premature failure. To prevent this problem, the stationary face must be installed perpendicular to the shaft and the rotating seal must be drawn up evenly with the gland, without being over tightened.
- Overheating: Overheating is often the result of a lack of lubrication. Signs of overheating are damaged “O” rings or stress cracks on the mating faces of the seal. Surface cracks are caused when the seal surface is exposed to intense heat due to friction of the mating surfaces. The metal beneath the surface is cooler than the surface, causing uneven expansion of the metal. Uneven expansion causes buckles and cracks in the surface, causing the edges of the cracks to raise. The sharp edges of the cracks cause rapid wear on the mating faces and eventually failure.
- Abrasive materials damage: If the fluid being pumped contains abrasive materials, these particles can work their way into the seal faces or wedge between the rotating seal and the shaft. Abrasion of the seal faces will eventually cause failure by leaving grooves in the mating faces. A buildup of particles forcing their way under the rotating seal will eventually cause the seal to freeze to the shaft, and if there is any wear on the mating faces, the seal will start to leak. Clean fluid should be used to lubricate mechanical seals whenever possible.
- Corrosion: If the fluid being pumped causes corrosion in the mechanical seal, it was most likely improperly selected for the application. Keep in mind, however, that a mechanical seal is normally made of a thinner material than the pump casing, impeller, or other components. It will be the first thing to fail if it is not compatible with the fluid being pumped. This could occur if the cooling system is being chemically cleaned and the system pump is used to circulate the chemicals.
Lubrication
All pumps require proper lubrication of the pump bearings, motor bearings, and the shaft seals. Follow the manufacturers’ guidelines for the proper type of lubricant to use and to determine the lubrication schedule. Do not overlubricate the equipment—it can cause overheating. Grease bearings should only be approximately 1/3 full. Oil- lubricated bearing reservoirs should always be filled.
This article is adapted from BOMI International’s course Boilers, Heating Systems, and Applied Mathematics, part of the SMA and SMT designation programs. More information regarding this course or BOMI International’s new High-Performance Sustainable Buildings credential (BOMI-HP™) is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.