Pump dampener webinar – ask the expert

Questions and answers from our webinar "Pump it up, damp it down"

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By Kelly Eberle, PEng, Principal Consultant

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The answers below were given in the context of the webinar and do not address all aspects of the issues discussed.
For more comprehensive information or application support, we strongly encourage you to contact the expert directly.

1

Calculations, modeling and guidelines

Are the dampener sizing formulas applicable to all types of dampeners?

The formula in the webinar is an example and for information only. Other types or manufacturers may have their own formula. In our experience, these formulae are too simple particularly for pumps > 50 HP and variable speed applications.

Do you have field data that provides validation of the pulsation model and the sensitivity to dampener size?

We have extensive field data validating results from our proprietary software. I do not believe we have field data that tests a single system’s sensitivity to different gas-charged dampener sizes.

Have you done the same or similar experiments with neck geometry and location effects?

We have not done these sensitivity studies for the example used in the webinar. It will likely be done for future presentations. The neck and gas charged bladder essentially form a notch filter or Helmholtz resonator, so the characteristics of the dampener are well understood.

Is it possible to transfer these criteria based on API 674 and empirical formulas to the API 618 base cases?

Since liquid and gas systems behave differently, dampener sizing criteria are different. The surge volume sizing equation in section 7.9.3.2 is more intended as a starting point for a vessel size as well as provide a consistent size for project proposals. The low-pass filter calculation in Appendix O is similar to the approach described in the webinar. We have a pulsation bottle sizing tool on our website to help determine vessel sizes for reciprocating compressors (API 618).

Is the dampener in your case study on the suction line or discharge line?

Suction side.

Is the difference between the empirical sizing and your model layout and pump-type dependant?

Yes, the pump type and system layout will affect pulsation results. In general, we find a similar relationship in that the empirical sizing always calculates reductions in pressure pulsations with an increase in dampener volume and the pulsation model does not show significant changes in pulsations with increasing dampener gas volume. We have found that a small amount of volume (gas charged dampener) will attenuate pulsation to a certain level and that additional volume will not attenuate pulsations further.

When we prepare a geotechnical report, sometimes we provide Gmax and damping values for a machine foundation. What do you recommend?

The geotechnical information and foundation response does not have a significant impact on the focus of this webinar, that being dampeners and the effect of the dampener size on the pressure pulsations, shaking forces and vibration of the piping. In the limit, there is a relationship between the geotechnical information and the support for yard or off-skid piping but it is not discussed in this webinar.

Where does the modelled "piping system" terminate or end? At the next piece of equipment?

The ideal location to terminate a model is at a large volume (i.e., separator, tank, large vessel).

2

Pump pulsation/vibration factors

How do fluid viscosity and density affect dampener performance or requirements?

Fluid viscosity does not have a significant impact on pressure pulsations and dampener performance. Fluid density is somewhat related to pressure pulsations. Density is a key term in pressure drop and line losses. Higher pressure drop due to line losses tends to reduce pressure pulsation. Fluid density is also related to the fluid bulk modulus or compressibility of the fluid. The bulk modulus is a key factor that defines the acoustic velocity of the fluid and therefore acoustic resonances. A greater or less dense fluid does not generally mean pulsations are significantly higher or lower, although less dense fluids tend to result in slightly more attenuation of pressure pulsation over a long distance as compared to more dense fluids.

How does the inlet velocity to a PD affect the pump in terms of vibration?

Regarding pressure pulsations and shaking forces, it is our experience that the inlet velocity to a PD pump has little impact on pulsations. Generally, higher velocity flow will result in more pressure drop which means damping or smoothing out of the flow fluctuations caused by the pump.

How does the length of the straight run of the discharge pipe before the first bend affect pulsation and vibration?

In our experience, there are no significant effects on pressure pulsations and vibration if a bend is located near the pump outlet. Perhaps there are effects from the flow not being fully developed or distributed across the pipe cross-section a short distance from the pump discharge. This non-distributed flow might lead to a local area of high velocity and perhaps erosion on the bend.

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What is the effect of foundation stiffness/damping and structural supports for piping on the results of pulsation models?

There is generally no influence of the foundation on the pulsation model, that is pressure pulsations and shaking forces generated by the pump. The foundation stiffness and damping properties for pipe supports and the pump package are important to consider. The key goal of the piping structural dynamic design is to achieve piping and pump package mechanical natural frequencies above 1.2x PPF.

The piping layout shown included multiple bends. In a simpler system with minimal bends, could cheaper pulsation options be used effectively on well-designed systems as less attenuation may be needed? Pulsation issues seem to be compounded by the piping design.

Yes, to a certain extent reducing the number of elbows is beneficial. Elbows are locations where the pressure pulsations act to cause shaking forces. Elbows are also locations of flexibility in the piping system that reduces the mechanical natural frequency and make the pipe more responsive to pulsation generated shaking forces.

Changing the pipe layout to reduce elbows can be thought of as a method of fine-tuning a piping system design to minimize vibration. Changing a pipe layout to reduce elbows will not reduce the underlying pulsation energy that the pump generates as part of its normal operation.

I expected that foundation sizing design issues (and providing geotechnical parameters for design such as Gmax and Damping) would be commented on since it has a bearing on vibration control - could this be addressed in the future?

The geotechnical information and foundation response does not have a significant impact on the focus of this webinar, that being dampeners and the effect of the dampener size on the pressure pulsations, shaking forces and vibration of the piping. In the limit, there is a relationship between the geotechnical information and the support for yard or off-skid piping but it is not discussed in this webinar.

3

Types of dampeners

I have used in-line pulsation dampeners (process fluid passes thru center of cylindrical bladder, N2 outside of bladder (line a pipe in pipe). This style of pulsation dampener was not included in your presentation. Do you have any thoughts on their advantages/disadvantages?

Unfortunately, we did not have time to discuss this design. We understand the design but have not seen it in practice in many applications. In theory it should work well as the fluid is fully exposed to the gas volume. Damage to the bladder from the flow may be a reliability concern.

Is it often the case that the empty volume dampener is the ideal solution? How does it compare in cost to other types?

Due to the complexity of pulsation analysis, there is not a blanket solution that works in all cases. Vessels and filters are not typically off-the-shelf type solutions, so they can be expensive. Depending on the pressure, the wall thickness can be large which increases the cost as well. The support structure beneath the vessel often requires modifications due to the size and needs to be considered.

The [“maintenance-free”] dampening vessel still requires inspection/integrity program, right? It is not necessarily maintenance free, right?

True, inspection is still required. The same inspection is required for gas charged bladder dampeners. This inspection may be required on a yearly, or longer, time frame. In comparison bladder type devices require frequent checks of the charge pressure and adjustment of the charge pressure if operating pressures changes. Also, the pump system is at risk of fatigue-related failures if the bladder leaks or fails and the bladder failure is not detected in short order.

What study should we carry out if we have gas-bladder damper already installed and to check if the dampers are effective or not?

A field study in combination with a pulsation analysis might be helpful in this situation. We would use field pulsation and vibration data to identify any existing issues. The field data can also be used to validate an as-found pulsation model that could then be used to evaluate any design changes.

4

Project management

Regarding getting early involvement of the pulsation specialist - what form of engagement is usual? How would this impact competitive bidding of pump packages - what sort of requirement would be in pump RFQ? Does pump vendor retain responsibility for overall system performance?

The responsibility of the overall project lies with the owner, not the pump vendor or packager. The owner must be properly engaged in the project and understand the issues at risk with an improper dampener design.

There are two approaches that the owner can use to ensure the proper dampener is picked and there is a level playing field for companies bidding on the package:

1. The owner prepares a specification requiring bidders to engage a pulsation specialist at key decision points.
2. The owner engages a pulsation specialist early in the project to define the dampener required and uses the result as the basis for competitive bids.

The consideration of using the consultant and timing is essential, in the life cycle of the project say at FEED. How soon after basic layout should I involve a consultant?

The sooner a pulsation specialist can get involved the better. Once basic performance data is available (fluid, power, speed, pressures, temperatures) we can start evaluating the type of solution that will likely need to be incorporated into the design. As further details become available, we can start evaluating the system with modeling.

5

Field validation

Is there a way to check if your damping system is working?

The only way to check if a gas charged dampener is working is to shut down and depressure the pump and dampener to measure the dampener pressure.

Do you have field data that provides validation of the pulsation model and the sensitivity to dampener size?

We have extensive field data validating results from our proprietary software. I do not believe we have field data that tests a single system’s sensitivity to different gas-charged dampener sizes.

6

Contact an expert

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Don't hesitate to contact our dedicated pump pulsation and vibration specialists.

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