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Reprinted from Feb 2016 LNG Industry

Simon Smith & Christopher Finley, Ebara International Corp., USA, explain why it is important to look at overall efficiency of submerged motor pumps and not pump hydraulic efficiency alone.

 

Introduction

Rotating Equipment Engineers who are accustomed to evaluating the performance of conventional centrifugal pumps using air-cooled motors, need to adopt a slightly different approach when evaluating the performance of submerged motor pumps.  Due to the mechanical differences between the two types of pumps and the inherent difficulty in analyzing pump and motor efficiencies independently, it is important to evaluate the combined pump and motor efficiencies, rather than pump hydraulic efficiency on its own.  This article describes a method used to evaluate overall system efficiency, allowing engineers to accurately compare products from various manufacturers on a level playing field.

Background

Submerged motor pumps (Fig 1) are widely used in the oil and gas industry for pumping cryogenic and low temperature fluids such as LNG, LPG, Ethylene, Ethane, Propane and Butane.  SMPs use a pump and motor that are mounted on a common shaft and are immersed in the process liquid – either in the storage tank itself or in a suction pot as shown here.

  • Submerged motor pumps provide several benefits over traditional, air-cooled motor pumps.  Some of these benefits are:
  • No mechanical seal or sealing systems required resulting in no risk of seal leakage
  • No potential for shaft coupling misalignment caused by the large temperature differential (for example, ambient temperature outside the vessel and -160OC inside the vessel)
  • Short rotor length results in classically “stiff shaft” rotor design, leading to lower vibration and long bearing life
  • Reduced noise as the motor is immersed in the fluid inside the vessel
  • Reduced equipment footprint as there is no external motor, sealing system or lubrication system.
  • Thrust is dynamically balanced so no external thrust bearing required

How Submerged Motor Pumps Are Tested

Conventional pumps are normally tested in water, using a calibrated test shop motor of known efficiency. Thus, from the overall absorbed power observed during the test, the pump hydraulic power and efficiency can be directly extracted and this is what is tabulated and guaranteed. The job motor is separately tested with a dynamometer and separately guaranteed.  Rotating Equipment Engineers evaluating two or more pump vendors therefore usually compare pump hydraulic efficiencies and powers in their bid tabulations.

With a submerged motor pump, the pump and job motor are tested together as an assembly in a cryogenic test fluid (normally LNG or LPG). The motor cannot be separately load tested due to the common shaft, thus the separation of the pump hydraulic power from the motor power is an inexact science.  There is anecdotal evidence that some pump manufacturers may artificially reduce motor efficiency and power factor values to make the pump hydraulic efficiency appear higher than it actually is (recognizing that Rotating Equipment Engineers have a tendency to evaluate competing products on pump hydraulic efficiency rather than overall efficiency).

For submerged motor pumps, pump hydraulic efficiency cannot be directly measured. It is a derived value obtained from:

  • Motor Current  (measured)
  • Pump Flow Rate (measured)
  • Pump Differential Head (measured)
  • Hydraulic Power (calculated)
  • Overall Efficiency (calculated)
  • Pump Efficiency (by deducting assumed motor efficiency from the overall efficiency)

Ultimately, rotating equipment engineers are only interested in the overall combined power consumption and current draw for the pump and motor together. This is what will determine the ongoing energy costs to operate the equipment as well as the required electrical infrastructure to support pump operation.  Separating out pump power or efficiency can lead to errors as can be demonstrated in the following example (taken from a recent real life situation).

Pump Efficiency Calculation Example
In this example, two SMPs from different manufactures are presented.  Both pumps operate at identical flow rates and differential pressures and are similar in construction.  The following table summarizes the general performance data provided by each pump manufacturer:

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At first view, it appears that pump B has a higher efficiency than pump A (72% vs 70%), and is therefore more desirable.  Upon further analysis, however, this may not be the case.  If we drill down a bit further, we see that the claimed motor efficiency of pump A is 89.2% at a power factor of .902, while the claimed motor efficiency of pump B is 85.9 at a power factor of .875.  Knowing the following relationships, we can use this information to determine the overall efficiency of the pump and motor combined as well as the power required to operate the pump.

 

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From this example, we see that although pump manufacturer B gives a higher value for pump efficiency (72% vs. 70%), pump A actually provides a more energy efficient solution (353 kW vs. 356 kW) by requiring less power consumption for a given duty point.

In addition to the power consumption of the pump, motor current draw is important for plant design to ensure proper switchgear and motor protection design.  For three phase induction motors, the motor current is related to both pump power and motor power factor by the equation:

 


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This example shows that even though the pump efficiency of pump B is stated to be higher than pump A, pump A actually requires less current to operate.

Conclusion

With more and more rotating engineers analyzing the performance of submerged motor pumps, it is important to look in depth at the actual overall efficiency of the pump and motor combined instead of the pump hydraulic efficiency alone.  When this method is used, pumps from various manufactures can be compared from an identical perspective, allowing a true measure of overall pump system efficiency.

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Also, please click HERE to check out our Cryogenic Centrifugal Pump Efficiency Calculator!