When sizing, selecting and installing hermetic pumps in refrigeration units, there are many considerations. This article will explore the types of pump as well as provide a guide for troubleshooting common problems.
Types of Centrifugal Refrigeration Pumps
Open-Drive Refrigeration Pumps. Open-drive refrigeration pumps have been utilized for decades in industrial cooling. Beginning in the 1950s, advancements in seal technologies and reliability — coupled with the increasing amount of frozen foods in consumers’ shopping carts — greatly increased utilization. These centrifugal pumps are used in applications like cold storage, food processing, refrigeration, ice production and turbine inlet cooling. Used as liquid overfeed and transfer systems, pumpages often employed include anhydrous ammonia, aqueous ammonia and halocarbons such as R22.
Open drives require a mechanical seal, but they have safeguards to avoid any emission into the atmosphere. Also, the oil-lubricated seals do not rely on the presence of pumpage to operate. They commonly come in discharges sized from 1.5 to 4” based on the system needs. The design of the open drive makes them a little more forgiving with materials in the pumpage. Having liquid as clean and pure as possible is always recommended, however. They typically can handle particles up to 0.375” in size.
One advantage is the seal system allows the pump to continue to run while the pump reprimes itself, avoiding nuisance-tripping conditions. Working pressures generally go up to 250 psig. While rotation speed typically can be up to 3,600 rpm, operation at 1,800 rpm or less may allow better NPSH characteristics.
Hermetic Pumps. Hermetic pumps are a newer design. Created for work in chemical and nuclear applications, hermetic pumps also have found a niche in refrigeration applications. They do not require a mechanical seal because the seal is integrated with the bearings. This allows for greater thermal protection (up to 392°F [200°C]), enhanced vapor-handling capabilities, built in-bearing monitoring and increased reliability in regards to pumpage release due to secondary containment. Like open-drive pumps, while rotation speed can be up to 3,600 rpm, operation at 1,800 rpm or less may allow better NPSH characteristics.
Refrigerant pumps are commonly employed in liquid-overfeed and transfer-pump applications utilizing anhydrous ammonia, aqueous ammonia, and halocarbons such as R22 and other approved refrigerants.
Centrifugal Pumps. Also known as a clean liquid overfeed pump, standard centrifugal pumps can be used in cooling tower, chilled water, glycol, brine, condenser spray-tree and HVAC applications. Clear liquid pumps come in more sizes than either open-drive or hermetic pumps, and they are operable over a wider range of conditions. When used as transfer pumps, they can move compressor oil, be utilized in liquid transfer systems or operate as booster pumps.
In many refrigeration applications, secondary coolants — for instance, ethylene and propylene glycols, or brines — are used as heat transfer media. The glycol or brine is cooled by the primary refrigerant and used to transfer heat without changing state. Centrifugal clear liquid-handling pumps are used to recirculate these secondary coolants. The clear-liquid pumps are rugged, efficient and designed for a long life of service.
Troubleshooting Hermetic and Open-Drive Refrigeration Pumps
Hermetic and open-drive refrigeration pumps share similarities in the common operational problems they encounter. The three most common issues are:
- Vapor Entrainment.
In querying distributors and maintenance operators, more than 90 percent of problems fall into those three categories.
Cavitation. When too much flow is demanded of the liquid-overfeed or transfer pump, cavitation will result. Cavitation is the formation of bubbles or cavities in the ammonia, glycol, brine or other pumped liquids. When the bubbles or cavities collapse, it causes pitting of the pump casing and impeller. Pumps undergoing cavitation are often reported to sound like they are pumping rocks or marbles. The damage to the pump can be severe and debilitating.
There are several steps to take if you are experiencing cavitation. If it is an acute instance, closing the stop valve should bring about immediate recovery. If the cavitation is chronic, it is likely that the net positive suction head required (NPSHR) is greater than the net positive suction head available (NPSHA). The system design should address this. The pump likely needs to run slower to improve the NPSHR (within factory tolerances).
Hermetic technology allows the pump to operate without the need for a mechanical-shaft seal.
Recirculation. Common to all centrifugal pumps, recirculation is condition that occurs when a pump is run significantly below the pump’s best efficiency point (BEP). On a pump curve, recirculation is prominent when the pump is operating below one-half to one-third of BEP (to the left hand side of a pump’s efficiency curve.)
Recirculation happens when secondary flow begins within the impeller, whereby the fluid actually reverses direction and exits the eye of impeller towards the suction (as opposed to flowing out of the discharge). In some cases, it even has enough energy to flow into the discharge and reverse course backwards towards the impeller. Turbulence and small vortices occur because of the recirculation. The high velocity at the core of the vortices results in low pressure of the fluid, possibly leading to cavitation.
What to do if the pump is experiencing recirculation? As noted earlier, closing the stop valve is effective in stopping cavitation due to excessive flow rate. However, if the pump is experiencing recirculation, shutting the valve will exacerbate the recirculation. In that case, the operator should re-open the stop valve and establish the minimum flow required via a bypass line.
Minimum flow requirements can be determined for a given refrigeration pump by following this procedure:
- Fully close the bypass line valve.
- Close the discharge stop valve.
- Slowly close the bypass line until the pump discharge pressure starts to become unstable as indicated by bouncing of the pressure gauge needle.
- Open the bypass line valve until the gauge stabilizes.
Vapor Entrainment. The phrase “vapor entrainment” implies that the vapor is carried into the pump from an external source. This is distinct from cavitation, where the vapor is actually generated within the pump.
Vapor entrainment is traditionally misinterpreted as cavitation. However, similar to cavitation, vapor entrainment may actually result in an auditory signal or diminish the amp draw by the motor. Moreover, vapor entrainment typically restricts the flow of the pumpage through the eye of the impeller — to the point that coils may not be adequately fed.
When vapor entrainment restricts the flow of the pumpage through the eye of the impeller, inclusion of a bypass line will not resolve this operational problem. However, if the pump is connected to an adequately sized and adjusted bypass line, or there are system loads online, then recirculation can likely be ruled out. Vessel and piping design are always involved no matter what is deemed to be the greatest contributing factor.
Semi-hermetic refrigeration pumps operate without the need for a mechanical seal using hermetic technology. This pump includes an integrated bearing monitor for continuous monitoring during operation.
Horizontal vessels always have less submergence available to separate the surface boiling from the mouth of the drop leg. Simple vortexing is not often a problem anymore because most vessel manufacturers have learned to include crossed-plate vortex eliminators or similar devices in the mouth of the drop leg. However, vapor entrainment due simply to the proximity of the boiling layer to the mouth will still occur if the submergence is less than about 18”. If any pressure drop in the vessel is quicker than 1 psi/min, boiling will occur about 3’ below the liquid surface. In such cases, obviously, 18” of submergence will not prevent vapor from entering the drop leg. In a vertical vessel, it would be unusual if there were not more than 3’ of submergence. However, if the drop leg is properly sized, then the full liquid height from the operating level to the pump level can be used to protect against vapor entrainment rather than just the submergence above the mouth of the drop leg.
Vapor-entrainment problems are also directly related to the rate of pressure drop during any transient in the vessel. This leads to the discussion of false loads. The key to avoiding vapor entrainment in a vessel is to keep the rate of pressure drop as low as possible during any pressure reduction. Pressure reductions occur when a system is started up from ambient, so the temperature and pressure must be brought down to their design values before they stabilize.
Pressure reductions also occur when something upsets the system. Possible upsets include increased refrigeration demand caused by a new batch of warm product being brought to a freezer, or increased shipping and receiving activity, which allows more warm air into the refrigerated space.
Another source of false load occurs as a result of hot-gas defrosting practices. During hot-gas defrost of an evaporator, the liquid-supply solenoid to that evaporator closes, the defrost regulator on the evaporator outlet closes, and a hot-gas supply solenoid opens. Hot gas from the high side enters the evaporator and warms the coils. After a while, the pressure inside the evaporator builds enough that the defrost regulator opens. As a result, hot gas starts blowing down the wet return line to the LPR.
In addition to the aforementioned recommendations, there are a variety of piping design recommendations considerations that can help minimize the effects of vapor entrainment. In particular, pay attention to the bypass line, pump-leg line velocities, suction vent line and volute line.
Overall, to reduce likelihood of vapor entrainment, keep the following in mind. Vertical vessels are much less likely to lead to entrained vapor than horizontal. If the system allows, consider vertical configuration. Reduce instabilities in process loads versus cold storage loads. This lessens the likelihood of boiling within the vessel. Boiling in the vessel increases the likelihood that the pump will ingest vapor, increasing the possibility of entrainment.
Proper sizing helps as well. Adequately size and adjust the bypass line. Design the vessel and piping with at least 2’ of NPSH available (more than the maximum NPSH requirement). Have a minimum of 18” of submergence to prevent vapor entering the drop leg. Size the drop leg to have a velocity near 25 ft/min, so bubbles will rise at a faster rate than the downward liquid velocity.
Operational techniques help as well. Reduce the amount and times that warm air and warm product enter the freezer. The increased demand on the system increases the likelihood that vapor will become entrained. Keep in mind that hot-gas defrosting can lead to vapor locking, and the plant will have trouble maintaining temperature. Use caution and be cognizant when employing this method. Finally, remember that at initial flooding, refrigerant absorbs heat from pump casing and can boil off. The gas produced tends to collect at the high point of the volute and must be vented off prior to startup. PC