Low Loss Pulsation Control

8/13/2019 Low Loss Pulsation Control

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GMC 2010: Low-Loss Pulsation Control Page 1

Low-Loss Pulsation Control

GMC 2010

By Marybeth Nored (SwRI), Buddy Broerman (SwRI), Klaus Brun (SwRI)

and Gary Bourn (El Paso)

Introduction

The advancement of reciprocating gas compressor technology for pipeline gas

transmission has been made possible, in part, due to the effectiveness of the compressorpulsation control system. The vast majority of current high speed compressors operate less

efficiently than low speed compressors due to the higher flow rates which lead to higher

associated flow related losses through the valves, orifice plates and the pulsation filter bottles.

Also, with higher horsepower machines operating over a large relatively high speed range (800-1200 RPM) pulsation amplitudes tend to be significantly higher, leading to higher dynamic

losses and high risk vibrations on the manifold and cylinders. Advanced pulsation controltechniques are needed to accommodate the increase in compressor horsepower and the range of

running speeds as well as the variation in operations.

The current GMRC pulsation control research program is developing concepts to controlpulsations more efficiently and effectively than the orifice plates and standard approach to filter

bottle design, used in the past. Two of the more promising and mature advanced pulsation

technologies developed by GMRC at Southwest Research Institute are the Pressure RecoveryInsert (PRI) and the Virtual Orifice (VO). Both devices can be used to control high pulsations

from a piping resonance, excited by one or more compressor orders. The two devices weredeveloped to reduce the high losses associated with orifice plates (commonly used as insertionplates to reduce the amplitude of high fluid pulses). However, the two technologies are designed

differently and mitigate pulsations through entirely different mechanisms. This paper will

review both technologies and field test case studies to show the benefit of these low-losspulsation controls.

Options for nozzle resonance control have been one of several areas addressed more

recently by the GMRC pulsation research namely because a large population of existing cylindernozzle orifice plates may be eliminated through advanced methods of pulsation control. The

nozzle resonance controls can also be implemented in a retrofit manner relatively easily, at a

lower cost than new filter concepts or system / station control concepts. The standard practicefor nozzle resonance control has utilized orifice plates to dampen the high pressure pulse at the

resonant frequency. This is one method of attenuating a resonant response which can be added to

the system design at a low cost. Its primary disadvantages are the pressure loss associated withthe orifice plate and a potential for increased pulsations in the lower frequencies (typically at 1x

and 2x compressor running speed).


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Modeling of Pulsations in the Gas Passage and Nozzle Area

Pulsation models of reciprocating compressor systems commonly utilize a one-dimensional (1-D) representation with acoustic length modifications to represent the three-

dimensional system, given the simplicity and cost-effectiveness of this approach. This has

historically been the approach to compressor cylinder modeling for the past 50 years. One-dimensional models are generally accurate for piping systems where the dominant physical

length is in the flow direction. In the areas near the compressor cylinder, very close to the fluid

force excitation from the piston, the 1-D assumptions break down since many of the high-frequency energy components have not diminished.

In the case of the cylinder nozzle responses for modern high speed, high horsepower

compressors, three-dimensional models are more appropriate. Without accuracy in bothfrequency and amplitude predictions of the related gas passage resonances, it is unclear whether

a significant acoustic resonance exists. In the past, it was possible to simplify the 3-D geometry

to a one-dimensional representation because the higher frequency responses were diminished in

lower horsepower machines. This is no longer the case for modern reciprocating compressors.It is very difficult to design pulsation controls properly without accuracy in the gas passage and

nozzle resonances in terms of both frequency and amplitude prediction.

The combination of a 3-D acoustic response model and a 1-D fluid representation model

must be utilized to provide accurate predictions of all gas passage system responses in a costeffective manner. Once the frequencies have been characterized correctly with the 3-D modal

analysis, the representative one-dimensional fluid model can be calibrated and can be used to

predict the significant amplitudes and attenuation mechanisms. The 1-D fluid model should be a

one-dimensional derivation of the Navier Stokes equations and include the non-linear termswhich can often affect the shape of the resonant peaks and the accuracy of amplitude predictions.

The recent SwRI refinement in combining 3-D acoustic modal analysis and 1-D fluid models hasgreatly aided SwRI in the ability to design the Virtual Orifice acoustically. Centering the VOabsorption frequency directly on the nozzle response is important for controlling the side band

responses, which will be present to some extent in any compressor.

The Virtual Orifice

The Virtual Orifice is an option for nozzle resonance control, which utilizes the concept

of absorption of the primary nozzle response frequency. The cylinder nozzle resonance is

actually a combined effect of the cylinder gas passage, cylinder nozzle, and primary volume

bottle. As such, it should be more correctly termed the passage-nozzle-bottle response. Thesethree component geometries in combination create an originating volume, a small passage way

and a final volume which produces an acoustic response that is typically close to the classic

Helmholtz type response. Since the gas passage and nozzle dimensions often have dis-similardiameters and lengths, the response is difficult to calculate by hand and must be simulated in a

fluid model (see Section 2 on modeling). The nozzle resonance is sometimes misrepresented as

a quarter wave acoustic response (with open-end, closed-end boundaries) but it is in typicallymuch closer in its behavior to a Helmholtz type acoustic response. See Figure 1 for examples of


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Figure 2. Development of t he Virtual Orifice for Nozzle Resonance Control

Field test measurements of the compressor pulsation, vibration, and performance were acquired inJuly 2007. Figure 3 shows the VO installed at the suction valve cap on one side of the compressor.

Pulsation measurements were obtained at a single cylinder valve cap for each cylinder. Data were

measured for the following configurations: (1) with neither the orifice nor the VO installed (baseline); (2)

with only the orifice installed, and (3) with only the VO installed. A summary chart of the pulsation

measurements resulting from testing each of the three configurations described above reveals the benefits

of installing the VO (see Figure 4). For configuration 1 with no controls in place, the cylinder nozzle

response was just above four times compressor running speed (4th order or 4x), and the maximum

pulsations resulting from that response placement peaked out at 52 psi [359 kPa]. Pulsation amplitudes at

2x increased significantly when the orifice was installed (configuration 2), resulting in a maximum

pulsation amplitude of 30 psi [207 kPa]. The large increase in 2x pulsations is indicative of an undersizedorifice. With the VO installed, maximum pulsation amplitudes over the entire 0 to 200 Hz frequency

range were reduced to 15 psi [103 kPa].


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Figure 3: VO installed at on Suction Valve Cap at El Pasos Baxter Station in Wyoming

Figure 4. Field Data Show Reduced Pulsations with VO Installed

In summary, maximum pulsation amplitudes over the 0 to 200 Hz frequency range were reduced

by 71 percent with the VO installed. Overall pulsations were approximately 50 percent lower with the

VO installed as compared with that of the orifice. This demonstration effectively moved the VO to a

Primary Suction Bottle

Compressor

Cylinder

VO

installed


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TRL 7 status, but due to the lack of field experience with the VO, additional field testing was warranted

on other units. The Baxter Station VOs was were installed on one valve cap of each cylinder of a two-

cylinder Cooper unit on a suction valve cap. Further testing was necessary to determine acceptability of a

discharge valve cap installation and the adaptability of the design to other cylinder types, other

manufacturers and various valve cap styles.

The most recent VO testing was performed in May 2010 for Boardwalk Pipelines Destin

Compressor Station. Design work for the Destin Station VOs initiated near the end of the

pulsation design study. Boardwalk was presented with the predicted pressure drop summary that

was associated with the recommended cylinder nozzle orifices, and it was decided that thosepressure losses were undesirably high. Therefore, Boardwalk decided to pursue a low pressure

loss alternative to the orifices, which was the VO.

The Ariel JGC/4 (4-cylinder, 1-stage) unit was installed at Destin Station without any

cylinder nozzle pulsation control devices, and VOs were installed after initial/baseline field data

was obtained. Data before and after the VOs were installed comprised of pulsation, vibration,

and performance measurements. For the performance assessment, both PV card and enthalpyrise measurements were obtained. As described in the following paragraphs, the data generally

showed that installing the VOs resulted in reduced pulsation and vibration amplitudes while

maintaining essentially the same compressor performance.

Field measurements summarizing the measured cylinder valve cap pulsation amplitudes

are shown in Figures 5 (suction) and 6 (discharge). Initial suction system data was taken withoutthe VOs or orifices installed, and it revealed a nearly 70 psi pk-pk resonance at approximately 58

Hz (on 4x compressor running speed). After installing the VOs, the 70 psi resonance went down

to less than 12 psi at 58 Hz, and a maximum pulsation amplitude of 24 psi was measured for theentire 0-500 Hz frequency range. Initial discharge system data was taken without the VOs or

orifices installed, and it revealed a 112 psi resonance at approximately 58 Hz (on 4x compressorrunning speed). After installing the VOs, the 112 psi resonance went down to less than 15 psi at

58 Hz, and a maximum pulsation amplitude of approximately 28 psi was measured for the entire0-500 Hz frequency range. Cylinder nozzle pulsation amplitude reductions were significantly

attenuated, and as described below, cylinder vibrations were consequently reduced.


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Figure 5. Field Data Showing Reduced Suction Valve Cap Pulsations with VO Installed

Figure 6. Field Data Showing Reduced Discharge Valve Cap Pulsations with VO Installed


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Similar to that of the pulsation measurements, significant cylinder vibration

improvements were also observed in the field study. A summary of some field measured

vibration is provided in Figures 7 and 8. A 58 Hz resonance is in the red (square data points)plot in the two figures that corresponds with the cylinder nozzle acoustic resonance note

previously. Installation of the VOs reduced the 58 Hz vertical vibration amplitude of 1 ips 0-pk

down to less than 0.2 ips 0-pk and the 58 Hz horizontal vibration amplitude of 0.3 ips down toless than 0.1 ips. Only minimal differences were observed for the vibrations in the stretch

direction of the cylinders. Overall cylinder vibrations were significantly improved with the

installation of the VOs.

Figure 7. Field Data Showing Reduced Cylinder Vertical Vibration with VO Installed


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Figure 8. Field Data Showing Reduced Cylinder Horizontal Vibration with VO Installed

Based on the efficiency values measured with the PV cards, the compressor efficiencywas reduced by approximately 3% when the VOs were installed. Based on the efficiency values

measured with the lateral-to-lateral temperatures and pressures (enthalpy rise method), theefficiency was reduced by approximately 0.5% when the VOs were installed (see Figure 9).

When taking into consideration the uncertainties associated with the efficiency measurements,the baseline efficiency is statistically equal to the efficiency with the VOs installed. That was the

case for both the PV Card and enthalpy rise methods. No appreciable loss in compressor

efficiency was observed when the VOs were installed in the baseline case system, which had nocylinder nozzle resonance controls installed. Another finding of the performance analysis was

associated with the measured power per unit flow (horsepower per MMSCFD). The horsepower

per MMSCFD indicated that installation of the VOs may have lowered the work requirement,possibly due to the reduced pulsations and dynamic pressure effects.


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Figure 9. Field Data Showing Essentially Unchanged Compressor Performance with VO Installed

Installation of the virtual orifices at Boardwalk Pipelines Destin Compressor Stationresulted in generally anticipated data. Installation of the VOs did not

result in an appreciable

loss in compressor efficiency compared to that of the baseline case, which had no cylinder nozzleresonance controls installed, but did result in significant reductions of pulsation and vibration

amplitudes associated with the compressor cylinder nozzle resonance. Cylinder nozzle pulsationamplitude reductions of 66-75% were observed. Cylinder vibration amplitude reductions of 66-

80% were observed. The virtual orifice has proven to be an effective replacement of cylinder

nozzle orifices that also results in a compressor performance boost.

Pressure Recovery Insert (PRI) Nozzle

Lower efficiencies and higher pulsations are more evident for high speed, high

horsepower machines due in-part to the higher mean flows and coincidence of the cylinder

nozzle acoustic response with lower compressor orders. The Pressure Recovery Insert nozzlewas developed to control nozzle pulsations by attenuating the pulsations through a choking

effect, similar to an orifice plate. The PRI mounts in the flow stream like an orifice plate

between the two flanges. However, this device recovers some of the initial losses from the flowrestriction through a low angled pressure recovery outlet. Figure 5+x (left and center) shows a

solid model and fabricated PRI nozzle. Figure 5+x (right) shows the installation of a suction side

PRI 8-inch diameter nozzle in August 2009 at the first PRI field test.


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The design depends on the beta ratio requirements and the allowable length for the

installation. Recent fluid modeling results (see Figure 6+x) from the GMRC research programindicated the sensitivity of the pressure drop to the design of the inlet and outlet fluid angles.

The upstream inlet path and the downstream recovery angle are designed custom to each

application to provide sufficient pulsation reductions but also reduce the separation andrecirculation as these flow disturbances significantly add horsepower losses to the system. Other

design aspects that need to be considered are material compatibility and pre-existing piping tap

holes. Depending on material requirements and the gas composition, the PRI nozzle can be

made from stainless or carbon steel using CNC fabrication or from rapid prototyping usingSelective Laser Sintering, which creates a stainless / bronze matrix. Testing is ongoing to

determine if pressure measurements can still be made (accurately) at a pressure tap which is

effectively measuring pressure on the back side of the PRI. For temperature measurements, theRTD probes must be adjusted to not overlap with the outside wall of the PRI nozzle.

Figure 5+x. Pressure Recovery Insert nozzle developed through GMRC pulsation research

Figure 6+x. Flow modeling for optimized design of the Pressure Recovery Insert nozzle

The first field site PRIs were installed in August 2009 at the El Paso Elk Basin station onthree first-stage suction nozzles of Unit 11. At this station, two units operate in parallel in two

stages of compression over a wide pressure ratio and flow rate range (42-66 MMSCFD). The

speed range is 240-300 PRM. The first stage PRI nozzles were installed to replace orifice plates,utilized to control pulsations in the range of 30-50 Hz. Figure 7+x shows the first stage suction

bottle and cylinders with the indicated location of the PRI nozzles.


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Figure 7+x. PRI installation on Unit 11 suction nozzle at El Paso Elk Basin Station

Test dates for were not available on Unit 11 until March 2010. At the request of El PasoPipeline Company, SwRI conducted a vibration and pulsation field study during the period of

March 23-24, 2010. SwRI recorded measurements of the vibrations and pressure pulsations at

the facility to assess dynamic characteristics. Measurements of the differential pressure betweenthe suction bottle and the suction valve cap were also recorded with dynamic pressure

transducers to assess pressure losses associated with the Pressure Recovery Insert (PRI) devices.The location of the dynamic pressure upstream and downstream taps is shown in Figure 7+x.PRIs were installed at the first stage suction connections on cylinders 1, 2 and 3. Testing was

also performed to verify the addition of a side-branch absorber volume to control 1x piping

pulsations in the inter-stage piping. Tests were performed with Unit 11 (with PRIs) and Unit 12

(no PRIs installed) at both double acting and single acting conditions.

SwRI modeled the piping and cylinders initially to determine the recommended pulsationcontrols. First stage suction nozzle pulsations were expected to be in the range of 10-12 psi pk-

pk at 33-38 Hz with no controls in place. The second order pulsations at 9 Hz were also

expected be in the range of 10-12 psi pk-pk. At the field site with the PRIs installed and for

double acting conditions, the pulsation amplitudes measured between 1.1 to 6.4 psi pk-pk,resulting in a reduction of approximately 50% in peak amplitudes compared to the SwRI

pulsation model (see Figure 8+x). The single acting case confirmed a similar reduction in the

nozzle pulsation amplitudes compared to the SwRI pulsation model.

Upstream DP tap

and bottle

dynamic pressure

Downstream DP

tap and valve cap

dynamic pressure

PRI located in

suction nozzle


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Pressure drop measurements were also taken to determine PRI pressure loss. SwRI

utilized high frequency pressure transducers to determine the average loss (associated with asteady flow situation) and the additional dynamic loss contributed by the pulsating flow. Total

pressure drop at the field site was measured at 0.85-0.99 psi from the suction bottle, through the

nozzle with the PRI in place, to the suction valve cap (including the gas passage). Steady flowpressure drop for the PRI would have been 0.13-0.15 psi. The difference between the steadyflow and total measured pressure drop includes the dynamic effects. The table shown below

reveals the calculation for the orifice plate pressure drop for one cylinder at 14.3 MMSCFD and

the expected pressure drop improvement. The PRI is expected to save approximately 1.5-3.0 psiper cylinder for a total horsepower gain (for the entire compression process) of approximately

0.7%. Additional savings are expected on each stage of compression.

Table 1. Pressure drop improvement / horsepower savings per first stage PRI

The field test data showed that the modifications to the piping system from SwRI 2009

analysis resulted in reduced pulsations and vibrations for all load conditions. The study also

demonstrated the use of the PRI device as an acceptable nozzle resonance control (instead of a

cylinder nozzle orifice plate). Due to the success of the first field test, SwRI has designed eightmore PRIs to provide a complete set of first stage suction and discharge and second stage

El Paso Elk Basin - First Stage Suction

qm(lbm/sec)= 7.325

Vel (ft/sec) 13.976Q(ft^3/min) 292.703

ReD= 1,770,824

Q (MMSCFD)= 14.383

OP perm press loss 0.689112941

Ventu ri perm ploss 0.150444417

(* Above Based on chart from Mil ler, p.6.42, reproduced)

Total Ploss improve per PRI 3.13

Q (MMSCFD)= 14.38

PRI HP gain per PRI 4.00

No. of cyl / PRI's in this stage 3

Horsepower recov total for stage 12.004

Total hp recov for unit 12.004Estimated total hp 1770

Percent horsepower recovery 0.68%

SwRI pulsation model predicted

10-12 psi pk-pk for nozzle resonant

pulsations


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discharge PRIs for Unit 12. Fabrication of these new PRIs (shown below) has been completed.

Testing is planned on Unit 12 for September 2010 after installation. The tests will compare the

Unit 11 performance (with some orifice plates in place) to Unit 12 (which is completely outfittedwith the new PRIs).

Summary

SwRI advanced pulsation control designs for the Virtual Orifice and the Pressure

Recovery Insert nozzle have both shown to control pulsations in the compressor cylinder nozzlebut without the pressure losses associated with typical orifice plates. The methods of modeling

the pulsations near the cylinder valve (sources) and mechanical / structural loads are critical to

designing these technologies successfully for different compressor cylinders and applications.Through the field test cases, these designs have proven to be robust and reliable. Both of these

advanced technologies possess over one year of field service and have been designed and tested

for multiple field sites on low speed and high speed machines.

References

[1] Broerman, E. , Bourn, G., McKee, R. and Nored, M., Benefits of the Virtual Orifice:Pulsations and Vibrations Reduced, Performance Improved, Gas Machinery Conference 2008.

[2] Brun, K., Development of a Transient Fluid Dynamic Solver for Compression System PulsationAnalysis, Gas Machinery Conference 2007, Dallas, Texas, 2007.

[3] Brun, K., Nored, M. G., Smalley, A. J., and Platt, J. P., Reciprocating Compressor Valve Plate Life

and Performance Analysis, proceedings of 4thConference of the EFRC, Antwerp, Belgium, June 9-10,2005.

[4] Deffenbaugh, D. M., et al, Advanced Reciprocating Compression Technology (ARCT), SouthwestResearch Institute Final Report, SwRI Project No. 18.11052, prepared for U.S. Department of Energy

National Energy Technology Laboratory, December 2005.

[5] Foreman, S., Compressor Valves and Unloaders for Reciprocating Compressors An OEMs

Perspective, Dresser-Rand Literature, 2002.

[6] Woollatt, D., Reciprocating Compressor Valve Design, Optimizing Valve Life and Reliability,Dresser-Rand Literature, 2003.

[7] Chaykosky, S., Resolution of a Compressor Valve Failure: A Case Study, Dresser-Rand Literature,2002.

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Low Loss Pulsation Control