The Electronic Newsletter of The Industrial Refrigeration Consortium Vol. 14 No. 3, 2014

Transcription

1 chan The Electronic Newsletter of The Industrial Refrigeration Consortium Vol. 14 No. 3, 2014 OIL MANAGEMENT DO S AND DON TS Oil management is one area of major difference between ammonia refrigeration systems and those systems using fluorochemical refrigerants like R22. The refrigerant-oil solubility in fluorochemical refrigerant systems means that both refrigerant and oil will circulate throughout the system. In these systems, provisions must be made in the design and operation to ensure oil returned to the compressors for lubrication. In contrast, the oils generally used in ammonia refrigeration systems have quite low solubility with the refrigerant. This means that any oil migrating outside of the compressor package will tend to settle out in the system accumulating in low points and usually requiring manual removal. In this edition of The Cold Front, we discuss both design and operating practices to effectively manage oil in IRC Staff Director Doug Reindl 608/ or 608/ dreindl@wisc.edu Assistant Director Todd Jekel 608/ tbjekel@wisc.edu Research Staff Dan Dettmers 608/ djdettme@wisc.edu In This Issue Standards Update 1-13 Upcoming Ammonia Classes 2 Noteworthy 2 John Davis jgdavis@epd.engr.wisc.edu IRC Contact Information Mailing Address Toll-free University Avenue Phone 608/ Suite 3184 FAX 608/ Madison, WI info@irc.wisc.edu Web Address

2 built-up industrial ammonia refrigeration systems. We review where and why oil is needed in ammonia systems as well as why we strive to prevent its migration from the compressor package itself to other parts of the system. System design practices intended to enable safe oil management are presented along with maintenance practices aimed at ensuring safe, efficient, and reliable ongoing system operation. Introduction Oil is a critical element needed to support the efficient and reliable operation of the positive displacement compressors used in industrial ammonia refrigeration systems. In the case of screw compressors, oil is used as a lubricant for bearings and it is injected into the compressor as a dynamic sealant to fill the tight clearances between the meshing rotors and the enclosing housing that envelops the rotor tips. As a dynamic sealant, oil helps minimize internal leakage of the refrigerant vapor being compressed; thereby, preserving high volumetric efficiency during the compression process. Oil is also used as a hydraulic fluid for actuating the movement of slide valves, slide stops or other hydraulicallyactuated control devices on the compressor package. In reciprocating compressors, oil is used to lubricate the pistons, connecting rods, and crankshaft. Like screw compressors, oil can also be used as a hydraulic fluid to actuate cylinder unloaders. Although the vast majority of oil Upcoming Ammonia Courses Introduction to Ammonia Refrigeration Systems October 8-10, 2014 Madison, WI Principles and Practices of Mechanical Integrity for Ammonia Refrigeration Systems November 5-7, 2014 Madison, WI Intermediate Ammonia Refrigeration Systems December 3-5, 2014 Madison, WI Process Safety Management Audits for Compliance and Continuous Safety Improvement January 12-14, 2015 Madison, WI Introduction to Ammonia Refrigeration Systems March 4-6, 2015 Madison, WI Ammonia Refrigeration System Safety April 13-15, 2015 Madison, WI Design of NH 3 Refrigeration Systems for Peak Performance and Efficiency September 14-18, 2015 Madison, WI Process Hazard Analysis for Ammonia Refrigeration Systems September 23-25, 2015 Madison, WI Noteworthy Visit Send the items IRC of website note for to next access newsletter presentations to Todd made Jekel, at the 2011 tbjekel@wisc.edu. IRC Research and Technology Forum. Mark your calendars now for the 2012 IRC Research and Technology Forum May 2-3, 2012 at the Pyle Center in Madison, WI. Send items of note for next newsletter to Todd Jekel, tbjekel@wisc.edu. 2

3 in an industrial refrigeration system is used for compressors, it is also used as a lubricant in some other subsystems such as bearings in open-drive liquid refrigerant pumps. Because the volume flow rate of oil through screw compressor packages is relatively high, a specialty device called an oil separator is used to remove oil from the compressor discharge gas stream. Most oil separators use three principles for removing oil from the discharge gas: impingement separation, gravity separation & filtration using coalescing elements. We go to great lengths to retain oil in compressors to avoid two (2) very undesirable consequences when it escapes out to the refrigeration system. First, the loss of oil from a compressor package will diminish the reservoir of oil available for continued use as a lubricant within the compressor package. Second, the lost oil from the compressor package will migrate to other parts of the refrigeration system to become a contaminant. Compared to a refrigerant such as ammonia, oil is an extremely poor heat transfer fluid and any oil accumulation within heat exchangers will result in a marked loss of heat transfer capability. Figure 1 shows an illustration of a screw compressor package with its oil separator. The oil separator is a large vessel that receives the discharge gas exiting the compressor. When the oil-laden discharge gas enters the separator, the gas-oil stream is directed toward the vessel s head. The momentum of larger oil droplets causes them to impinge on the head where they are removed by impaction. The larger cross-section area of the oil separator dramatically decreases the gas velocity and as the gas moves laterally through the separator, Motor Compressor Discharge vapor Compressor discharge Oil Separator Oil 1 st Stage Oil Separation 2 nd Stage Oil Separation Figure 1: Typical screw compressor oil separator. 3

4 intermediate size oil droplets simply fall to the bottom of the vessel by gravity. The very small droplets of oil continue to be carried along by the refrigerant vapor flow where they proceed to flow through a second stage of separation. In this stage, the refrigerant vapor-oil stream is forced through a series of coalescing filter elements. The coalescing filters increase the surface area available to intercept the smaller misty droplets of oil. Ideally what leaves the oil separator is high pressure ammonia vapor that is free of oil. Because no oil separator is 100% effective, a small amount of oil will leave the oil separator and migrate out to the system. The goal is to minimize, to the greatest extent possible, oil carryover from the oil separator. The two (2) biggest factors that can lead to excessive oil carryover are (1) increased gas velocity through the oil separator (both during steady operation and transient conditions) and (2) compromised function of the coalescing filters. The velocity of gas through the oil separator is dependent on the mass flow rate of refrigerant (capacity) through the compressor and the density of discharge gas. If the gas velocity through the oil separator is higher than planned for in the original design, an increase in oil carry-over will occur. The following are examples of changes that increase the refrigerant gas velocity through the oil separator: Operating with suction pressures higher than the limits identified in the original design o Increasing suction pressure significantly increases compressor capacity and increase compressor capacity results in higher mass flow rate and thus higher gas velocity Operating with discharge pressures lower than the limits identified in the original design o Decreasing discharge pressure also increases compressor capacity but the capacity increase is minor. The bigger effect is the decreased gas density in the oil separator causing increased gas velocity. Operating with an excessive refrigerant flow rate for liquid-injection oil cooling (increases mass flow rate in oil separator) Running the compressor at a higher shaft speed than the original design (i.e. increasing the capacity of the compressor) Table 1 below illustrates how changes in the discharge pressure and suction pressure impact the volume flow rate of gas through the compressor s oil separator. Because the cross-sectional area of the oil separator is constant, the changes in volume flow translate directly to changes in the gas velocity. Higher velocity means lower separation effectiveness. As the high-side pressure decreases from the design condition of 166 psig to 115 psig, the velocity of discharge gas through the oil separator at full-load condition will increase by 41%. Approximately 5% of that increased gas velocity is attributable to the compressor s increase in mass flow rate the majority of the increased gas velocity is due to the decrease in gas density at the lower discharge pressure. 4

5 Table 1: Effect of changes in suction and discharge conditions on thermosiphon oil cooled compressor performance. P suction (psig) T suction ( F) P discharge (psig) T discharge ( F) Capacity (tons) Mass flow (lb/min) Volume flow (CFM) Volume flow (% change) Baseline % % In addition to the above-mentioned conditions, there are short-term transients that often cause excessive oil carry-over. The most common condition is a rapid decrease in the compressor s discharge pressure. When a rapid drop in discharge pressure occurs, the density of refrigerant gas in the oil separator decreases which results in a proportional increase in gas velocity. The increased gas velocity degrades the oil separator s ability to remove oil from the refrigerant and increased oil carryover results. Rapid decreases in discharge pressure on the high-side can be caused by poor condenser capacity control during cool weather conditions (e.g. cycling water pumps on and off for head pressure control) and high instantaneous hot gas demands for defrost (too many evaporators simultaneously calling for hot gas or defrosting large evaporators without a soft-gas step that gradually allows the pressure to build in the evaporator reducing the rapid in-rush of gas at the start of the defrost cycle). Apart from the above-mentioned operating conditions that can cause increased oil carryover, there is also are also maintenance issues that can enable oil carryover. Consider the second stage coalescing filters themselves. Generally, a multiplicity of these filter elements, similar to the one shown in Figure 2, are needed for the second stage oil separation. If the filters themselves are not properly seated and sealed, discharge gas can bypass the filter media and increased oil carryover results. The intent of the discussion regarding oil separation thus far is to increase your awareness of common factors that cause increased oil carryover. The key is to avoid known conditions that can increase oil carryover and when oil carryover is suspected take immediate steps to address the causes. Figure 2: Coalescing filter element. Preventing oil migration out of the compressor will minimize the adverse effects it can have on low-side performance as well as the need to deal with oil that has left the compressor packages. 5

6 Oil Management We have already discussed the purpose of oil and the goal of preventing oil from leaving the compressor package. How then do we deal with or manage oil that does manage to escape the compressor package(s) and migrate out to other parts of the system? For the vast majority of industrial ammonia refrigeration systems, oil management on the low-side of the system involves installing points in the system where it is intentionally allowed to accumulate and then monitoring those points so the oil can be manually removed on an as-needed basis. The subsystem designed to collect the oil is called an oil pot. IIAR 2 includes the following requirements relating to oil management: 14.2 Oil Removal Provisions shall be made for removing oil from piping and equipment where oil is likely to collect Oil removal shall be accomplished by one or more of the following: a. A rigid piped oil return system. b. A vessel equipped with an oil drain valve in series with either a self-closing or manual quickclosing emergency stop valve connected to the oil drain point, a vent line, a vent line isolation valve, and an approved pressure relief device. c. Piping which provides capability for isolation and refrigerant removal to another portion of the system. d. An oil drain valve in series with a self-closing or manual quick-closing emergency stop valve. When draining to atmosphere, rigid piping routing the oil 2 to 4 ft [0.6 to 1.2 m] away from and within sight of the valves shall be provided. Use of temporarily attached rigid piping and emergency stop valves is permitted. e. Any other suitably engineered system. The rigid piped oil return system option identified in (a) typically consists of a central oil accumulator vessel located in a machinery room (or outdoors) and a dedicated piping system designed to convey accumulated oil from various points out in the system to a central oil accumulator vessel where it can be safely removed from the system and subsequently recycled. In this design, the oil return process is accomplished by operators actuating manual valves to allow oil to flow from the various drain points to the accumulator. Oil Pots The majority of plants with industrial ammonia refrigeration systems utilize the option described in (b) 6

7 where the vessel referred to is an oil pot. Figure 3 shows a schematic for an oil pot installation connected to a recirculator package. The oil pot itself is a small vessel with its supply line piped from a low point on the recirculator. Because oil is heavier than liquid ammonia, it will tend to collect in the low points of a system. In this case, oil that finds its way into the recirculator will collect at the bottom of the vessel. With the liquid refrigerant supply pipe to the pump physically stubbed higher into the bottom of the pressure vessel, a small volume to collect oil in the main vessel is created. The liquid supply line to the oil pot is then connected so that any accumulated oil along with liquid refrigerant will flow by gravity to the oil pot. Because the oil pot is uninsulated, heat gain from the ambient environment causes the colder liquid refrigerant within the oil pot to evaporate and return back to the pressure vessel through the oil pot s vent line. The oil is nonvolatile at these low temperatures and collects at the bottom of the oil pot. The intent of an oil pot is to provide a means of isolation from the system so that oil can be safely drained. Oil pot vent Recirculator Oil pot supply Oil pot drain Oil Pot Figure 3: Recirculator package with oil pot. 7

8 Figure 4 shows a more detailed schematic of the oil pot itself including its key valves. During normal operation, the liquid drain valve (V-101) is open and liquid ammonia containing any accumulated oil flows from the main vessel will flow to the oil pot. The liquid ammonia within the oil pot will absorb heat from the surrounding environment and evaporate. The ammonia vapor produced during this evaporation process then flows back to the vessel through the vent line because the vent valve (V-102) is normally open. The oil drain valve (V-104) and the quick-close valve (V-105) are normally closed. Most oil pots have the vent pipe stubbed down into the pot slightly. This arrangement creates a small vapor pocket in the top of the oil pot to minimize the likelihood of liquid ammonia flowing out the relief valve (V-103) if it were to weep. Figure 5 shows a photo of an oil pot installed on a low-temperature recirculator. The frosted portion of the oil pot s surface is the portion of the oil pot that contains low temperature liquid ammonia. The refrigerant vapor that collects at the top of the pot is relatively poor at transferring heat so the rate of heat gain absorbed from ambient environment is sufficient to prevent frost from forming on the top of the pot. The melted frost at the bottom of the oil pot is due to the presence of collected oil. As oil continues to collect in the pot, the frost level will progressively rise. It is desirable to drain oil prior to the pot completely filling so operators will use the progressive disappearance of frost on the pot as a visual cue to schedule the pot for oil draining. Valve Type Normal Position During Oil Draining V-101 Shut-off Open Closed V-102 Shut-off Open Closed V-103 Safety relief Closed Closed V-104 Shut-off Closed Manually regulating V-105 Self-closing Closed Open V-103 V-102 V-101 V-105 V-104 Refrigerant vapor Refrigerant liquid Oil Figure 4: Oil pot details. 8

9 Vapor ammonia Liquid ammonia Oil Figure 5: Various levels shown on an actual oil pot installed on a recirculator. Oil Draining So it s time to drain oil what s next? Prior to draining oil, the oil pot must be properly prepared prior to executing the oil drain. CAUTION: Read, learn, train, and follow your plant s written operation/maintenance procedures for oil draining on your specific process equipment! Knowing that you will have plant-specific procedures for your oil draining, here we discuss key steps that should be consistent with your established procedures. Let s look at the steps involved and discuss why we proceed in this fashion. Referring to the valve nomenclature shown in Figure 4, the steps involved in oil draining are as follows: 1. Close liquid supply valve (V-101) This first step is intended to prepare the oil pot for draining by ensuring the liquid ammonia is evaporated out of the oil pot. When we close the liquid supply (V-101) to the pot while leaving the vapor vent valve (V-102) open, we are allowing ambient heat to evaporate the residual liquid ammonia within the pot. 2. Wait With the liquid supply valve closed, we let a sufficient time elapse to allow the ambient heat gain an opportunity to fully evaporate the residual liquid ammonia resident in the oil pot. The complete melting of frost from the outside of the oil pot gives us a visual indication that the residual liquid ammonia has evaporated and vented back to the accumulator. Usually 24 hours is sufficient to 9

10 complete the evaporation process. Vol. 14 No. 3, Gather PPE, oil receptacle, and other plant-required authorization/permits Generally, plant procedures will prescribe the PPE operators are required to have accessible or donned during oil draining. These may include chemical resistant gloves, apron, and safety glasses with face shield (or full-face respirator). Some plants have developed safe work practices such as line-break permitting. For these plants, oil draining likely falls within the scope and permits or other forms of authorization would be required prior to oil draining. Once these details are addressed, the actual oil drain process can proceed. Figure 6: Operator with plant-required PPE donned performing an oil drain. Note the oil pot is not frosted during the actual oil draining. Also note the second operator in the background behind the oil pot serving as a backup. 4. Close vent line valve (V-102) This step now fully isolates or segregates the oil pot from the system. 5. Position the drain receptacle to collect oil and carefully remove plug from oil drain line The plug in the oil drain line is intended to prevent any spillage of oil while the pot is in its normal service. Often there is some residual oil from the prior drain so proceed with caution and care in removing the plug. 6. Open the self-closing deadman valve (V-105) When the self-closing deadman valve is opened, there should not be any flow of oil from the pot 10

11 because the oil drain shutoff valve has not yet been opened. 7. Carefully open the oil drain valve (V-104) While manually holding the deadman valve open, the oil drain shut-off valve is now slowly opened to initiate the flow of oil from the pot. The oil drain valve is used to manually regulate the rate of oil flow from the pot to the receptacle - opening more to increase flow and closing more to decrease the rate of oil flow. If there is an unexpected increase in oil flow i.e. it spits, the self-closing deadman valve can be released; thereby, quickly stopping the oil drain flow. The oil flowing during the draining process often appears brown in color and is a bit frothy as shown in Figure 7. As mentioned above, the type of oils commonly used in ammonia refrigeration systems have low (but not zero) solubility with ammonia. Figure 7: Oil drained from pots is often chocolate in color and frothy. As the oil level in the pot decreases, the pressure of the residual ammonia vapor in the pot will also decrease. If the connected recirculator is operating at a pressure above atmospheric, the recirculator can be used to build-up pressure in the oil pot. This is accomplished by closing down on the oil drain valve (V-104) until it is just cracked open followed by releasing the self-closing deadman valve (V-105) to fully stop the oil flow. The vent valve (V-102) can now be opened momentarily to allow vapor from the recirculator to re-pressurize the pot. Re-close the vent-valve (V-102) and resume the oil draining by re-opening the deadman valve (V-105) and regulating the flow with the oil drain valve (V-104). The oil drain process is complete when the oil flow begins to be intermittent with vapor from the pot. Carefully place the oil receptacle in a well-ventilated area to let any ammonia vapor absorbed in the oil to off-gas. 8. Securely close the oil drain valve (V-104) while maintaining the self-closing deadman valve (V-105) in the open position The intent of maintaining the deadman valve (V-105) in the open position once the oil drain valve (V- 104) is closed is to enable any oil/vapor contained in the line between these two valves to flow out. 9. Open the vent valve (V-102) 11

12 10. Open the liquid supply valve (V-101) Vol. 14 No. 3, Carefully crack open the deadman valve (V-105) once again to verify the oil drain valve (V-104) is properly holding. If the oil drain valve is holding, then reinstall the cap or plug in the oil drain line. If the oil drain valve does not appear to be holding, retighten the drain valve until secured. Crack open the deadman valve (V-105) once more to release any residual oil and then reinstall the cap or plug in the oil drain line. 12. Log the quantity of oil drained Once the swelling of the oil has gone down following the off-gassing of ammonia, determine the volume of oil drained. The location of oil removal, the quantity of oil removed, and the date oil was drained should be noted in an oil log in accordance with Section 6.2 of IIAR Bulletin 110. Periodically, the quantity of oil added to the system s compressors should be compared with the oil drained to ensure there is not a net accumulation of oil in the system. 13. Dispose of the drained oil The oil drained from points out in the system is not suitable for reuse in the compressors. This oil is not only particulate-contaminated but will also contain water at concentration levels above thresholds acceptable for the compressors. The drained oil should be properly recycled. Do s and Don ts Managing oil in an industrial ammonia refrigeration system is not rocket science but it does require planning, care, and attention to detail particularly during the process of draining oil. Do: Check compressor oil levels in your routine daily rounds. When adding oil to a compressor ensure that you o o Confirm the oil being added is the correct for that particular machine Log the date, time, quantity of oil added, along with the unique compressor identification in your oil log Troubleshoot increased compressor oil consumption immediately when it is suspected. Use your plant-specific procedures for oil draining. If items in this edition of the Cold Front provide information to improve your oil draining procedures, update your procedures using your management of change (MOC) process as required by the PSM Standard. Retrain on revised procedures. Adhere to your plant-required PPE and associated safe work practices Ensure all technicians engaged in oil management are appropriately trained Positively isolate oil pots from connected vessels or other equipment when draining oil to atmosphere do not drain oil live. If this cannot be accomplished, either hard-pipe the oil return system or utilize a temporary movable vessel to receive the oil. 12

13 Fit your vessels or other equipment with provisions for oil draining in accordance with the requirements of Section from IIAR 2. Figure 8 shows both a high-side vessel (left) and a low-side vessel (right) with inadequate provision for oil draining. DON T Figure 8: Examples of vessels without adequate provisions for oil draining. CAUTION: DO NOT DRAIN OIL LIVE (i.e. not isolated from system pressures)! References IIAR 2, Equipment, Design, and Installation of Closed-Circuit Ammonia Mechanical Refrigerating Systems, with Addendum B, International Institute of Ammonia Refrigeration, Alexandria, VA (2008). IIAR 110, Start-up, Inspection and Maintenance of Ammonia Mechanical Refrigerating Systems, International Institute of Ammonia Refrigeration, Alexandria, VA (2008). 13