In a process plant industry, the reliability and effective availability of equipment are the prime requirements to having high productivity and for rendering faithful service to the consumers. The reliability of equipment must be ensured and probably could be estimated as following.
* By proper equipment selection: 60 percent
* By proper installation: 15 percent
* By proper operation and maintenance: 25 percent
The following case study describes a reliability problem with a Liquified Petroleum Gas (LPG) handling pump in a field and the way it was troubleshooted and fixed.
These pumps were installed and commissioned in March 1999. The mechanical seal started to leak intermittently at the interval of 45-60 seconds. This problem was persisting even after increasing the restrictive/regulating orifice (RO) size to 5 mm from 3 mm. (See Figure 1.)
Observations at the site included the following.
* Seal was leaking intermittently
* Suction pressure: 7.5 kg/cm2(g)
* Discharge pressure: 16 kg/cm2(g) (steady)
* Motor load: 55 amps (steady)
* Vibrations: 6.2 mm/sec max
Some of the suspected probable causes of intermittent seal leak included the following.
* Hung up rotary head assembly.
* Distortion in seal faces.
* Axial float in rotor.
* Seal chamber pressure below vapor pressure and liquid flashing into vapor.
After dismantling, the following observations were made.
* Shining marks observed on carbon face. Rotary face found good and intact.
* Elastomers found in good condition.
* Both bearings found good and intact. No axial float observed in the rotor.
* Impeller found intact.
* Axial movement of rotary unit found unrestrained and free of sticky deposits.
* Seal faces checked for flatness and found to be ok.
* Pump impeller was found to have a back wear ring, but no balancing hole was provided.
* The rotary head assembly was found free on the sleeve. Probable cause #1 was ruled out.
* Seal faces flatness found within two bands. Probable cause #2 was ruled out.
* No axial float observed in the rotor. Probable cause #3 also was ruled out.
* The shining marks observed on carbon face revealed that there was a loss of lubricating film on the mating faces, which could be due to the formation of vapors in the seal chamber that were not getting out from the chamber due to the too close clearance in the throat bush. This accumulation of vapors may be due to the heat generated at mating faces and dead-ended sealing chamber. The seal chamber bush clearance appeared to be insufficient to flush out the liquid vaporization due to heat generated by the seal faces, especially due to the fact that cooling water initially was not supplied.
The fact that the seal was failing intermittently with the periodic opening of seal faces and release of LPG into the atmosphere seems significant. This could be happening because the accumulated vapors (due to phase change from liquid LPG to gas) gradually would increase in volume filling out the seal chamber. Then, when the pressure would build up sufficiently, the faces would open up, causing the vapor to be released and the cycle to repeat.
Applied Solution and Recommendations
At first, changing to Plan 13 was considered. For LPG / Propane services, which have a narrow margin between suction pressure and vapor pressure at operating temperature, seal flushing Plan 13 was thought to take the excessive built-up vapors from the sealing chamber back to suction. This plan would consist of flush line from the seal chamber through flow regulating orifice (RO) to suction. However, it was decided that this would not solve the problem of vaporization in the seal chamber, because the seal box pressure then would be even lower than when using Plan 11, and there would be even less margin between the box pressure and vapor pressure. Thus, the Plan 13 idea was rejected.
The history of attempted modifications includes the following steps.
Step 1. Since the LPG service (the seal chamber pressure and heat generated by the rotating seal faces) is so close to vapor pressure, it is important to dissipate the heat generated at seal faces to avoid rapid vapor formation at the seal area. Initially, it was assumed that this was due to vaporization inside the seal box. The orifice size was increased in order to increase the seal box pressure. Unfortunately, this did not solve the problem.
Step 2. The API 610 8th Edition Cooling Water Plan 61 specifies the "tapped connection for purchasers use. Typically used when the purchaser provides fluid (steam, gas, water, etc.) to an auxiliary sealing device."
This initially was not connected as the location did not have cooling water available. This was discussed with the manufacturer, pointing to the fact that the LPG service may be sensitive to heat generation in the seal chamber. The manufacturer felt, however, that the pump may not require additional cooling, and so the provisions for the cooling water availability were not made. With a problem persisting, this needed to be addressed, and cooling water circulation through the sealing chamber jacket then was provided by hooking up the inlet and outlet lines to distanced headers of the neighboring unit without the need to dismantle the pump. Unfortunately, this did not solve the problem either.
Step 3. A 5-mm hole was drilled in the upper portion of the throat bushing. At the same time in the impeller, four balancing holes also were added. It was discovered upon disassembly and examination that they were missing. This modification worked, and the pump now is running satisfactorily without any leaks.
The seal box pressure for this type of a pump (after modification with back wear ring and balancing holes) is a suction pressure of more than 35 percent of discharge pressure as per manufacturer’s rule of thumb for the light hydrocarbons.
8.0 kg/cm2 + 0.35% ¥ 16 kg/cm2 = 13.6 kg/cm2 (g)
Interestingly, this almost is equal to box pressure before modification (as used by the "90 percent rule" calculation above). Theoretically, this modification does not change the seal box pressure substantially. What changes, however, is the amount of liquid from the discharge connection to the box and then through the opened-up bushing. In other words, the restrictive factor was the bushing clearance and not the orifice in the Plan 11 piping.
Hence, by making the hole in the throat bush and adding balancing holes, the passage for circulation of fluid carrying out frictional heat became less restrictive and solved the problem without excessive downtime and cost. The light hydrocarbon service pumps with a narrow margin between vapor pressure and suction pressure should be provided with a 3-5 mm drilled hole in the top portion of the throat bushing to allow the vapors to vent away from the seal chamber.