Introduction
In the process industry, equipment reliability and effective availability are the primary requirements for high productivity and faithful service to consumers. Equipment reliability must be ensured, with the following approximate importance:
By proper equipment selection (approximately 60%)By correct installation (approximately 15%)By proper operation and maintenance (approximately 25%)
The following case study describes a reliability problem with a liquefied petroleum gas (LPG) transfer pump on site, and how it was troubleshooted and repaired.
History
The pump was installed and commissioned in March 1999. The mechanical seal began to leak intermittently every 45-60 seconds. This problem persisted even after increasing the RO (restriction/regulating orifice) size from 3 mm to 5 mm.
Legend 1 From the high pressure area of the pump (pump outlet or pump outlet pipeline) 2 Flushing port (F) 3 Quenching port (Q) 4 Drain port (D) 5 Seal chamber Figure 1: Detail of standard seal flushing Plan 11
Plan 11 (including orifice plate) provides seal flushing; Plan 61 provides backup quenching, but no cooling water is provided initially. According to the manufacturer's instructions, the estimated pressure in the seal chamber is about 90% of the pump outlet pressure, so it is about (0.9×16)14 kg/cm2(g).
Service: LPG pump Manufacturer: Khimline Pumps, Ltd Rated flow: 76 m3/h Rated head: 173 m Inlet pressure: Design = 8 kg/cm2(g); Minimum = 7 kg/cm2(g); Maximum = 10.5 kg/cm2(g) Operating temperature: 50℃ Vapor pressure: 6.8 kg/cm2(g) 50℃ Specific gravity: 0.56 NPSHA: 2.5 mNPSHR: 1.9 m Radial bearing: NU310 Thrust bearing: 7310 (back-to-back installation)
Mechanical seal type: Single-end bellows mechanical seal with fluororubber elastomer. The seal balance ratio is 70%, and the pressure gradient coefficient of low-specific gravity liquid is 0.3. According to the manufacturer's instructions, the elastomer pressure is 0.45 kg/cm2. Sealing surface material: Carbon/SiC The axial hole on the bushing is not there at first (see the discussion below). Flushing plan: API Plan 11 (3 mm orifice plate), and reserved quench interface (Plan 61). However, cooling water was not provided initially, and interfaces F/D were plugged with screw plugs.
On-site operation
Intermittent leakage of mechanical seal.
Inlet pressure: 7.5 kg/cm2(g)Outlet pressure: 16 kg/cm2(g) (stable)Motor load: 55 A (stable)Vibration: maximum 6.2 mm/s
Possible causes of intermittent seal leakage
1) The dynamic ring assembly is in a suspended state2) The seal surface is deformed3) The rotor floats axially4) The seal chamber pressure is lower than the vaporization pressure, and the liquid flashes into steam
After seal disassembly
1) Flash marks are observed on the carbon surface, and the rotating surface is intact2) The elastomer is in good condition3) Both bearings are intact; no signs of rotor axial floating are observed4) The impeller is intact5) The axial movement of the rotating unit is not restricted, and there is no sticky deposit6) Check the flatness of the seal surface and find that there is no Question 7) It was noted that the pump impeller was equipped with a rear wear ring, but no balancing holes were provided
Analysis
a. Since the dynamic ring assembly was found to be free on the sleeve, the possible cause #1 was eliminated.b. There was no problem with the flatness of the seal surface, so the possible cause #2 was eliminated.c. No signs of rotor axial float were observed, so the possible cause #3 was eliminated.d. Flash marks were observed on the carbon surface, indicating that there was a loss of lubrication film on the mating surface, which may be due to steam formed in the seal cavity, and the steam could not be discharged smoothly from the seal cavity due to the small clearance at the throat bushing. This accumulation of steam may be due to the heat generated by the mating surface and the closed seal cavity. The seal cavity throat bushing clearance does not seem to be sufficient to carry away the heat of vaporization due to the heat generated by the seal surface, especially since Plan 61 did not initially provide cooling water.
Intermittent failure of the seal is inevitable as the seal surface opens periodically and LPG is released to the atmosphere. This may be because the accumulated steam (phase change from liquid LPG to gas) will gradually increase the volume of the filled seal cavity, and then, when the pressure is sufficiently accumulated, the seal surface will be opened, resulting in steam release. After the pressure is released, the seal face will be closed for a short period of time until the pressure rises to a certain value, and the seal face will be opened again, and the cycle will repeat.
Solution
Initially, the seal flushing plan was considered to be changed from Plan 11 to Plan 13. For LPG/propane applications, the difference between the suction pressure and the vapor pressure at the operating temperature is small, and Plan 13 is considered to be able to bring the excess accumulated vapor in the seal chamber back to the suction of the pump. This plan will include a flushing line from the seal chamber through the RO (flow regulating orifice) to the pump suction. However, this does not solve the problem of vaporization in the seal chamber because the seal chamber pressure will be lower than when using Plan 11, so the margin between the chamber pressure and the vapor pressure will be smaller. Therefore, Plan 13 was rejected. History of modifications/improvements:
Step 1 Due to LPG service (the seal chamber pressure and the heat generated by the rotating seal face are very close to the vaporization pressure), it is necessary to take away the heat generated by the seal face to avoid rapid vapor formation in the seal area. Initially, it was thought that this was due to evaporation in the seal chamber, and the orifice size was enlarged to increase the seal chamber pressure. Unfortunately, this did not solve the problem. Step 2 Plan 61 in API 610 standard is a reserved (backup) flushing solution, and its interface is plugged with a screw plug before the equipment leaves the factory. The site can finally decide whether to activate Plan 61 based on actual needs. Initially, it was not activated because there was no cooling water available at this location. This was discussed with the manufacturer, pointing out that LPG applications may be sensitive to heat in the seal chamber. However, the manufacturer believed that the pump may not require additional cooling and therefore did not make provisions for the availability of cooling water. As the problem still persisted, in order to solve this problem, cooling water circulation through the seal jacket was provided by connecting the inlet and outlet pipelines to the remote header of the adjacent unit without disassembling the pump. Unfortunately, this did not solve the problem either. Step 3 A 5 mm hole was drilled in the upper part of the throat bushing. At the same time, since disassembly and inspection revealed that 4 balancing holes were missing on the impeller, 4 balancing holes were added to the impeller. This modification was successful and the pump is now running well without any leakage. Based on the manufacturer's experience with light hydrocarbon applications, the seal chamber pressure of such pumps (after using the rear wear ring + balancing holes) ≈ suction pressure + 35% of the outlet pressure, that is:
8.0 kg/cm2+0.35%x 16 kg/cm2=13.6 kg/cm2(g)
Interestingly, this is almost equal to the seal chamber pressure before the improvement (as used in the "90% rule" calculation above). Therefore, in theory, this improvement does not significantly change the seal chamber pressure. However, what changes is the amount of liquid flushing from the outlet to the seal chamber and then through the orifice throat bushing. In other words, the limiting factor is the bushing clearance, not the orifice plate in the Plan11 pipeline.
Summary
By opening the throat bushing and adding balancing holes to the impeller, the fluid flushing circulation channel that transmits friction heat becomes less restricted and solves the problem without excessive downtime and cost. Experience shows that for pumps used in light hydrocarbon applications where the margin between vaporization pressure and suction pressure is small, a 3 mm-5 mm hole should be opened at the top of the throat bushing to allow steam to be discharged smoothly from the seal chamber.
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