Reciprocating pumps emit pulsations into the attached piping systems causing potentially dangerous unbalanced shaking forces, poor valve dynamics, high maintenance costs, and reduced flow. It is well known that gas charged elastomeric bladder or diaphragm pulsation dampeners can be used to effectively reduce the risks of these pulsation induced problems. Gas charged dampeners are relatively compact in size, and are easy to install when compared with maintenance free liquid filled options which tend to be large and more expensive to fabricate.
However, gas charged dampeners come with some drawbacks:
- Several failure modes: loss of gas charge, incorrect gas charge, elastomeric rupture, ineffective location.
- Must include maintenance activity to inspect the integrity of the elastomeric component.
- Must ensure the appropriate charge gas volume/mass in the cavity for optimum pulsation control.
- The potential for the dampener itself to become a vibration problem due to its branch connection being a heavy cantilevered mass having low mechanical natural frequencies.
- Ability to control pulsation only at a narrow range of pressure, flow conditions, and pump speeds.
- The lack of indication that the dampener is performing as it should.
These drawbacks are common issues for many operators. In a recent brownfield upgrade pump installation, Shell requested that the new pumps include a monitoring system to indicate when the gas charged dampener has failed. Beta Machinery (Beta) were contracted to include this study with the pulsation analysis being performed as per API 674. The hypothesis of the monitoring study was simple: when the dampener fails pulsations increase, an alarm is then triggered indicating a dampener failure.
The test of this hypothesis is presented in the following two-part case study of a brownfield upgrade. In the first part, we will provide the background of the oil company’s experience and the pulsation system model piping design. In next month’s installment, we will examine the complications that arise in a real installation and the challenges to overcome with variable speed pump operation.
BACKGROUND OF THE PROBLEM
Shell’s experience includes numerous reciprocating pumps for glycol and hydrocarbon condensate duties on both manned and unmanned offshore platforms. A majority of the older installations have reciprocating pumps equipped with bladder type pulsation dampeners. There have been numerous technical integrity issues related with the pulsation dampener losing pre-charge frequently, cases with the bladder material rupturing prematurely, and many cases of a notable increase in the vibration of the pump pipework leading to a failure and loss of containment.
With no form of indication available to ascertain the pre-charge pressure, it is difficult if not impossible to determine if the bladder, and/or bladder pressure is intact and holding. The only way to tell if the pulsation dampener is not performing as intended and has lost pre-charge is visual inspection of pipework. When the dampener has lost its pre-charge, the pipe work rattling and high vibration indicates some flaw in the working of the dampener. Under these circumstances the equipment has to be shutdown to manually inspect the dampener, as this has a direct impact on the reliability and downtime of the equipment.
The reciprocating pumps at the Shell Operating Unit facility are not equipped with on-line condition monitoring and the maintenance philosophy revolves around periodic off-line vibration condition monitoring typically carried out using a hand held instrument at three monthly intervals. For unmanned platforms the increase in vibrations are detected when the pump pipework vibration has increased substantially due to loss of pre-charge or the bladder giving away, presenting a high risk threat for operations.
The bladder in the dampener is very sensitive to changes in gas volume. As the line pressure changes, the volume of gas in the bladder changes, thus altering the system performance. The effectiveness of the pulsation dampeners can be reduced by bladder stiffness, bladder permeability, restriction of bladder expansion and contraction by dampener internals and degradation of performance due to large variations in line pressure, making the prediction of dampener performance complicated.
Technical integrity issues with the bladder material rupturing prematurely also surface when technicians charge the pulsation dampeners too quickly, leading to low temperatures at the elastomer interface and cracks propagating through the material. This has the effect of ultimately causing premature failure of the bladder material within a few charges.
Loss of containment through excessive pipe vibration and dampener failure, and the resulting impact on reliability statistics provided the motivation to address some of the issues described above. An opportunity to monitor the technical integrity of pulsation dampeners arose through brownfield project of F13 condensate transfer pumps (Triplex –double diaphragm type) to be installed on the existing E11 PB platform.
ATTEMPTS TO ADDRESS THE PROBLEM
Type of Dampener
In order to address the pulsation dampener integrity issues, a change in the type of dampener from bladder type to the acoustic liquid filled type was considered. This approach was successful only on the greenfield projects. For the brownfield applications this approach met with a limited success considering the fact that space and cost of the liquid filled dampener is much higher than an equivalent gas filled dampener.
The existing piping arrangement is often congested, particularly for offshore facilities, hence the option of installing a liquid filled dampener is effectively ruled out. Moreover most of the pumps within the facility are slow speed with low frequency. Thus this approach was not effective in resolving the brownfield installations.
Metallic bellows type dampeners were next considered for cases where the bladder failed prematurely or failed to hold the pre-charge with the elastomeric element slipping off. This approach did provide some amount of success especially with the high temperature glycol service where the fluid temperature is around 248 degrees Fahrenheit (120 degrees Celsius). The high temperature for the glycol service led to the premature failures of the bladder elastomeric element and cases where the bladder lost its pre-charge within a few days of operation.
Despite resolving some of the integrity issues the problem of not being able to detect the condition of pre-charge pressure for nitrogen remained.
Monitoring the Dampener Itself
Manufacturers of pulsation dampeners were requested to provide some form of indication on the pulsation dampener for ascertaining the pre-charge of nitrogen. Some of the pulsation dampeners complied with this requirement by providing a T-connection with one end of the T connected to a pressure gauge. However, such indication is purely local and is of value only at manned platforms. For the unmanned platforms the issue of pre-charge pressure uncertainty remained.
With appropriate signal conditioning, accelerometers could be used to measure the low frequency and high frequency vibration. Accelerometers mounted on the pump manifolds serve this purpose. The acceleration data can then be used to identify issues with plungers/control rods or valves. However, vibration on the pump piping is generally due to the unbalanced forces in the piping from pulsation, as such measuring vibration would be an indirect method of gauging pulsation. So why not measure pulsation directly?
Based on the limited success with the above approaches, Beta was engaged to work with a manufacturer to provide a solution for triplex pumps to be installed on an unmanned facility. The objective of this exercise was to design an effective and well controlled pulsation system, and at the same time develop a monitoring system capable of detecting the condition of pulsation dampeners and alert operations for any abnormal behavior in the system. It is also to provide some form of assurance to predict the behavior of the dampeners under dynamic variations in the system pressure. F13 condensate transfer pumps were the subject of this approach.
In the second part of this analysis, we will illustrate that the difficulty and risk in maintaining a safe pump system with these dampeners is magnified in offshore and unmanned equipment operations.
Until better options appear on the market to address these important issues, pulsation monitoring is a technique that can be used to monitor dampener integrity. ■
ABOUT THE AUTHORS
Jordan Grose is the manager of pump systems for Beta Machinery Analysis. Ravindra Pai is a senior rotating engineer at Shell. For more information, visit www.betamachinery.com. This data was presented in an altered form at Dusseldorf’s International Rotating Equipment Conference in 2012.
MODERN PUMPING TODAY, April 2013
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