34G-20A/B Oily Water Sump Pump Investigation due to Low Performance Indication

I. BACKGROUNDThe main function of 34G-20A/B is to pump the oily water from oil water concrete sump to the plant 34. The source of the oily water concrete sump is from the water sewer in utilities II (behind the boiler area). As per the performance test that was conducted at 19th May 2015, there were some problems regarding the stuffing box both in 34G-20A/B. The vacuum pressure at the suction of the pump makes the pump isolation (stuffing box) shall be very thigh. Currently, the stuffing box cannot isolate tightly. It makes the atmospheric air going inside the pump and the pump cannot pump the oily water properly. Secondly, the pump is self-priming type, and the self-priming system wasnt work properly and also need to be investigated. To keep the oily water not overflowing to the utilities water sewer, this pump shall work properly. Thus, the investigation is needed to solve the problems of the pump.

II. OBJECTIVEThe objective of this study is to identify a problems of 33G-20A/B, and recommend the effective solutions to solve the problems.

III. DISCUSSION

1. Priming PumpThe term "self-priming pump" describes a centrifugal pump that can use an air-water mixture to reach a fully primed pumping condition. A centrifugal pump is any pump that uses centrifugal force to create a pressure differential in a fluid, thus resulting in pumping action. Air is the main problem of a standard (non-self-priming) centrifugal pump. When the standard centrifugal pump encounters air, it can become air-bound. It is quite harder to pump air than to pump water, so when the air "binds" the pump, the pump can no longer force the water out. The minimum level of priming water need to be maintained to cover all impeller area. The fully and not-fully covered pump can be viewed as below.

Figure 1. (left) Fully Covered Priming Start and (right) not-Fully Covered Priming StartA self-priming centrifugal pump overcomes the problem of air binding by mixing air with water to create a fluid with pumping properties much like those of regular water.

Figure 2. Self-priming Pump while priming, pumping and at rest

The pump casing must be filled with liquid before the pump is started, or the pump will not be able to function. If the pump casing becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping. To ensure that a centrifugal pump remains primed and does not become gas-bound, most centrifugal pumps are located below the level of the source from which the pump is to take its suction. The same effect can be gained by supplying liquid to the pump suction under pressure supplied by another pump placed in the suction line (priming tank).

2. Stuffing Box MechanismA stuffing box is an assembly that is used to house a gland seal. It is used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements.

Figure 3. Nomenclature of Standard Stuffing Box

GlandA gland is a general type of stuffing box, used to seal a rotating or reciprocating shaft against a fluid. The most common example is in the head of a tap where the gland is usually packed with string, which has been soaked in tallow or similar grease. The gland nut allows the packing material to be compressed to form a watertight seal and prevent water leaking up the shaft when the tap is turned on. The gland at the rotating shaft of a centrifugal pump may be packed in a similar way and graphite grease is used to accommodate continuous operation. The linear seal around the piston rod of a double acting steam piston is also known as a gland. It is not always possible to use a standard centrifugal pumps stuffing box to seal the shaft of a centrifugal pump. The pump suction may be under a vacuum so that outward leakage is impossible or the fluid may be too hot to provide adequate cooling of the packing. These conditions require a modification to the standard stuffing box.One method of adequately cooling the packing under these conditions is to include a lantern ring. A lantern ring is a perforated hollow ring located near the centre of the packing box that receives relatively cool, clean liquid from either the discharge of the pump or from an external source and distributes the liquid uniformly around the shaft to provide lubrication and cooling. The fluid entering the lantern ring can cool the shaft and packing, lubricate the packing, or seal the joint between the shaft and packing against leakage of air into the pump in the event the pump suction pressure is less than that of the atmosphere.Figure 4. Stuffing Box Nomenclature Water Sealing/ Flush WaterEither the seal must be designed for the operating conditions in the rotating equipment or the environmental conditions surrounding the seal must be controlled within the seals design limits. When seals and their environmental controls are properly designed, installed and maintained, the results are well worth the effort in terms of safety, emissions, process efficiency, reduced materials, and seal life. To keep stuffing box working well, the pump is commonly equipped by flush water point as a secondary sealing mechanism beside the mechanical seal.

Figure 5. Standard Water Sealing Mechanism

The mechanical seal may be combined with the water sealing to keep the pressure inside the suction eye vacuum and also to keep the leakage from the fluid spilled out to the environment. The water sealing also act as a cooler of mechanical seal, and reduce the wearing rate mechanism in packing rings and high temperature in shaft due to the friction. The sealing fluid is not always a water, sometimes it is required a high viscosity fluid. But for simple mechanism the fluid can be circulated from the system itself.

3. Foot ValveFoot valves are a type of check valve and are placed at the pumps wet well. Unlike other valves, a foot valve is created with a larger flow area than the actual pipe size to make sure that there is less head loss. Foot valves are either made of PVC plastic or stainless steel, and they are known for keeping the continuous presence of suction within the pump. Foot valves are used to maintain hydraulic pressure to keep the water flow in accordance with the given settings or configurations. There are instances where the pressure can actually pop the valve out and cause major leakage; thus, it is important to use the right kind of material in the tubing to be able to support the force within the valve. One of the common type foot valve can be viewed as figure below.

Figure 6. Foot Valve IV. ANALYSIS

Figure 6. Performance Test of 34G-20A/B Test Report and Pump SpecificationOn 19th May 2015, FPE, MHE and Operation conducted the test to investigate the problem of 34G-20A/B, from the test we conclude the problem of the 34G-20A/B as below,1. Pump Priming Mechanism Failure 2. Atmospheric air leak indication 3. Low flow rate indicationAs per design specification, the pump can describe as below: Tag: 34G-20A/B

Service: Oily Water

Type : Self-Priming Centrifugal Pump

NoVariableValueUnit

1Capacity 17m3/h

2Temperature Max38C

3Discharge Pressure 0.41kg/cm2 (g)

4Suction Discharge-0.57kg/cm2 (g)

5Diff. head10.1m

6NPSHr2.3m

7NPSHa3.9m

8Speed1460rpm

9Water SealingReady, with 3/8 connection

Pump Priming Mechanism FailureThe priming mechanism failure could be driven by a single factor. The factor is when casing not fully covered by liquid (oily water) but it is filled by mixture of liquid and gases. When the pump is started the gas-bound effect makes the pump incapable to pump the fluid properly. The passing check valve (foot valve) made the pipe in the suction line filled with gas, and the priming tank incapable to handle the self-priming itself. In other the words, the trapped gas mixed with the fluid in the tank and makes the gas-bound effect that appears in the pump. The gas bound effect makes the suction eye pressure of pump higher than expected (not vacuum). Therefore, the liquid was not sucked to the section eye and the self-priming mechanism is unsuccessful. The passing check valve is exacerbated by improper sealing of stuffing box. Atmospheric air Leak Indication The indication of atmospheric air leak can be witnessed at the drained fluid that contained bubble. The bubble was an indication the sealing mechanism at the stuffing box not working properly. The vacuum pressure at suction eye naturally will suck the air from the atmosphere to the involute of pump. The sucked atmospheric air could increase the presence of gas-bound effect while the pump started, and this leakage will aggravate the load of the pump. The leakage would be an effect of high clearance of packing ring due to wear or high operating temperature.

Figure 7. Bubble in drain line (discharge pumps side)Refers to the API 610 10th Edition, the existing mechanism is API Plan 02. To cover leakage from the mechanical seal, the simple modification is API Plan 11. The API plan 11 is a product recirculation from pump discharge to seal through flow control orifice. The features of this plan are to prevent product from vaporizing by maintaining the pressure above vapor pressure, becomes a self-venting plan for horizontal pumps and as reinforcement of mechanical seal.

Figure 8. API 610, Plan 02 for Primary Flush Plan

Figure 9. API 610, Plan 11 for Primary Flush Plan

The stuffing box pressure should follow below equation:Stuffing Box Pressure (kg/cm2)= Pump Suction + 25% of the Total Dynamic Head= 0.41kg/cm2 + 0.25 * 1.01 kg/cm2= 0.6625 kg/cm2

The API states that the API Plan 11 should use flow control orifice as flow control. The flow control actually is to maintain the pressure of stuffing box. Due to the flow control is quite expensive, the flow control orifice can be substituted by the pressure indicator and valve. Therefore the valve can be adjusted to reach the required pressure of stuffing box. As per specification data sheet, the pump is already equipped with the 3/8 inch flush connection, and the piping connection shall follow CB2B PTB standard.

The modification of 33G-20A/B can follow these picture below:

Figure 10. SolidWorks Model of 34G-20A/B

Figure 11. SolidWorks Model of Flush Plan

Low flow rate indication As per test, the discharge pressure of the combined pump is 0.6 kg/cm2 and the discharge pressure as per specification sheet for single pump is 0.41 kg/cm2. As per data sheet the discharge pressure of combined pump should be 0.82 kg/cm2. The decreased performance at rated capacity should be affected by the wearing mechanism at the impeller and added by the chain reaction of check valve failure and sealing mechanism failure.

Figure 12. Discharge Pressure of Combined Pump at Rated Capacity

V. CONCLUSIONThe incapable of pumping at 34G-20A/B is affected by these three possibilities1. Self-priming mechanism failure due to passing check (foot) valve and exacerbated by improper sealing of stuffing box.2. The atmospheric leakage due to mechanical sealing failure. 3. The low performance is due to chain reaction of check valve failure and sealing mechanism failure and the impeller wearing.

VI. RECOMMENDATIONThe study recommends applying modification to solve these three problems as below:1. To replace the check (foot) valve with the new one, the type of valve is similar with the existing one. 2. To change water sealing mechanism from API 610 Plan 02 to API 610 Plan 11, and maintain the pressure of stuffing box at 0.6625 kg/cm2 to improve the sealing mechanism, as cooler and lubricator of packing rings.3. Internal pump checking to check the possibility of impeller wear, if the head does not reach the minimum pressure required at rated after performing the modification.

Prepared by EYP