During a mechanical seal replacement of a major gas plant, a reliability engineer identified that the pump was operating below Minimum Continuous Stable Flow. To resolve this issue, one company engineered modifications to the casing and impeller.
Dr. Gary Dyson, Hydro Global Engineering Services
Angelo Miller, Hydro Rocky Mountain
Eddie Janson, John Crane
Published by: World Pumps
Vibration commonly causes pumps to operate at low flow and reliability plays a key role in the cost effectiveness of systems. The pump in question was an additive booster pump. The seal manufacturer contacted Hydro to collaborate. Hydro’s field technicians identified that there was excessive vibration which increased wear on the mechanical seal.
Hydro plotted the current conditions on the pump curve and determined that the pump was operating below MCSF for which performance factors such as efficiency, rotor dynamic stability, bearing and mechanical seal life become negatively affected, yielding a drastically reduced Mean Time Between Repairs/Failures (MTBR/F). The company performed an OH2 power-end upgrade and a pump hydraulic rerate.
The pump service company then proceeded to perform case volute modifications in addition to manufacturing a new lower flow impeller. The company modified running clearances to modify the original best efficiency point (BEP) of the pump, which resulted in the current operating condition to rise above Minimum Continuous Stable Flow and at a more desirable percentage to BEP, producing an increased MTBR/F.
Furthermore, the existing bearing frame assembly design did not meet current API specifications for increased bearing and mechanical seal life, whereas the supplier’s bearing frame upgrade is designed to fully comply with API-610 specification. Conforming to these specifications provides an enlarged seal chamber and a larger diameter shaft which accommodates the API-682 compliant mechanical seal, extending the life of the mechanical seal and bearing.
The supplier recommended a rerate of the pump performance for current operating needs as well as an API compliant backend. Hydro estimated a turnaround of 10 weeks.
Causes of vibration
It is commonplace for pumps to operate at low flow. This is inevitable in many cases as the nature of many process operations place varying demands upon the equipment. When operating at low flow, a machine is under a much greater amount of stress than at most other operating flows.
To understand why operating at extreme low flows can cause pump problems, it is imperative to analyze the nature of the flow through a machine. Using computational fluid dynamics, the pump supplier can model the common flow picture of partial flow operation.
From a hydraulic design standpoint, vibrations at vane pass frequency are always inherent, due to how impellers generate head. Each vane has a pressure differential between the top and under side of the vane. This pressure differential results in a velocity difference over both surfaces of the vane.
This difference propagates through the passage out into the collector. Thus, the collector and anything connected to it will experience a fluctuating velocity field once per impeller blade pitch. These result from the turbulent ‘wake’ generated at the trailing edge of the blade, which is fueled by the two different velocity streams merging.
At partial capacity, this flow picture greatly deteriorates, as viewed in Figure 3.
Each passage is filled with slow moving fluid. The diffuser passages are stalled and the velocity is heavily time dependent. Operated in this condition, it’s clear how vibration and heat can become a problem for pumping reliability.
Inlet back flow
This phenomenon is always present within a pump operating at low capacity, and always contributes to the vibration level. High-energy liquid is expelled from the impeller eye. This expelled liquid dominates the suction passage, occupying two thirds of the flow area from the pipe outer diameter downwards towards the channel centerline. This flow spirals helically down the periphery of the suction pipe.
Each impeller blade generates individual streams. The inner third of the suction channel area has a spiraling core of slower moving fluid. Flow is dominated by the axial component of the velocity and tangential forces exerted by the peripheral flow. This causes the inner helical flow angle to be approximately double the outer angle. Furthermore, the helical spiral angle remains constant .
In theory, these velocity streams should not be responsible in the computational analysis for a vibration component. However, this is not always true in a real situation. Pump suction and piping designs contain discontinuities such as splitters, elbows and reducers. The velocity streams impacting on these features contribute to the vane pass frequency vibration.
The pump is already a low specific speed machine and this brings with it several challenges. This type of machine has its best efficiency point (BEP) flow dominated by the casing throat area. In practice this means that small changes to the casing throat result in quite large changes in the BEP flow, whilst changes to the impellers discharge do not make a much impact on BEP.
By making changes to the impeller eye, engineers can reduce the intensity of the inlet backflow and hydraulically generated vibration. This can be achieved by installing a specially sculpted ring into the eye of the impeller to invert the inlet backflow streams inwards into the impeller, thus preventing them from exiting the impeller at high velocity.
This solution can also be achieved by redesigning both the impeller eye and inlet vanes to ensure that the design changes become a permanent feature of the new impeller design rather than a modification.
After the seven week mark, the company was finished with their upgrade of the pump. Three weeks early, they completed:
1. Engineering design of volute insert with new throat area.
2. Engineering of impeller eye/new impeller.
3. Manufacture and weld in volute insert.
4. Manufacture new impeller/modify impeller.
5. Back pull out manufacturing and supply.
6. Pump assembly.
7. Maximum diameter performance test.
8. Final trim performance testing.
OH2 power-end upgrade – compliant with API-610 11th edition
New major components consisting of:
• Bearing housing (carbon steel)
• Case cover (carbon steel)
• Pump shaft (4140 ht)
• Case cover wear ring (420 ss)
• Throat bushing (410 ss)
• Isomag brand isolators
• Configured for oil ring lubrication – provisions for purge or pure oil mist lubrication.
• Case cover designed to accommodate api-682 double seal requirements.
• Case spiral gasket.
• Drive end frame cooling fan.
• New thrust and radial ball bearings.
• Assemble pump and power end upgrade completion.
• Install new mechanical seal and air test per company specifications.
In addition to completing the upgrade in a 7-week time frame, the pump company was able to fully test the upgraded pump at the test lab with the client present.
The pump was returned to the gas plant, and met its BEP in the field. In fact, the pump could operate at a lower energy rating while still outputting enough flow to maintain the needs of the gas plant.