Piping Design Considerations for Reciprocating vs Centrifugal Pumps

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Piping Design Considerations for Reciprocating vs Centrifugal Pumps Ramin Rahnama, Project Engineer Jordan Grose, Manager, Vibration Integrity Group BETA Machinery Analysis A Wood Group Company

Calgary Pump Symposium 2015

Presenters Ramin Rahnama, PEng

Ramin is a Project Engineer leading Design and Field teams in BETA’s special needs projects, as well as developing tools for field vibration analyses. He received his Master’s degree in Mechanical Engineering, specialized in Machine Dynamics, from the University of Calgary in 2010. Ramin has been with BETA for more than five years. He has extensive experience in machinery system vibration design by performing mechanical, pulsation, performance, and thermal stress analyses. He has also been leading vibration troubleshooting and maintenance projects for pump and compressor packages (on- and offshore) in many different countries. Besides his academic publications on vibration analysis in micro machining units, he co-authored “Improved Thermal Piping Analysis for Reciprocating Compressor Piping Systems” for the Gas Machinery Conference in 2012.

Jordan Grose, PEng

Jordan leads BETA’s vibration integrity department in addressing vibration, reliability, and integrity issues on rotating machinery piping systems, including pulsation, mechanical analysis, water hammer transient studies, small bore piping analysis, Energy Institute programs. He is a Mechanical Engineering graduate from the University of Calgary with a wide range of domestic and international design, field, and monitoring experience with compressors, pumps, and other production machinery. He has specialized skills in vibration, performance, and troubleshooting; and significant international experience in on- and offshore production facilities. Jordan has been with BETA Machinery Analysis for 12 years and was responsible for establishing BETA’s Malaysia office. He has authored, co-authored, and presented several articles and technical papers on rotating machinery and piping topics.

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Contents 1. Introduction: Centrifugal vs. Reciprocating Working Principles 2. Piping Design a) Centrifugal Pumps b) Reciprocating Pumps: Pulsations

3. Pulsation Considerations a) Pulsation Resonance b) Pulsation Dampener c) Cavitation

4. Mechanical Considerations a) Piping Layout b) Clamps c) Pipe Supports

5. Case Studies 6. Conclusion

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Key Points 1. Differences exist between centrifugal & reciprocating pumps, and their behavior while interacting with the piping system 2. Reciprocating pumps have many additional piping design considerations, not needed for centrifugals 3. Pulsation must be considered in recip pump piping design 4. Mechanical piping supports must be considered 5. Small bore piping vibration must be considered in the design stage 6. Recip pumps can work well, if they are designed well!

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Introduction

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Pump Categories Pumps

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Working Principles Centrifugal

Reciprocating

Virtually Continuous Flow

Intermittent Flow Calgary Pump Symposium 2015

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Reciprocating Pumps - Applications • • • •

High pressure (over 3000 psi) High viscosity Metering Non-Newtonian Fluids

Characteristics

• • • • • • •

Oil & gas Refineries Water treatments plants Cryogenic Food processing Fracking Pharmaceutical

Industries

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Piping Design Centrifugal vs. Reciprocating

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Centrifugal Pump Piping Systems LOADS

Static

Thermal Dynamics- Minimum At steady state conditions, vibrations and pulsations are typically not severe

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Standards • API 686 RP – No Tee or elbow near suction nozzle – NPSH calculations – No High point – Straight pipe to avoid turbulences

• API 610 – Nozzle forces and moments – Corrosion resistance – Pressure rating – Orifice size

No specification on piping supports or pulsations since dynamic loads are comparatively less!

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Reciprocating Pump Piping Systems

LOADS

Static Thermal Dynamics - Large

 Pulsations-induced forces up to 10,000 lbf

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Pulsation in Reciprocating Pumps

Calgary Pump Symposium 2015

Pulsation-Induced Unbalanced Forces Force (lbf) = Pressure (psi) * Pipe Area (in²)

Mean

Forces Cancel out

Dynamic

Pulsation (time varying)

Pressure

Pm + P

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Acts on elbows & change in geometry

Pm - P

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How Pulsations Cause Failures • Pulsations in piping does not necessarily mean failures will happen… Pulsation

Forces

Vibration

Stress

Failure

• Forces, vibration, and stress caused by pulsations can be a problem Calgary Pump Symposium 2015

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API 674/675 (for Reciprocating Pumps)

Design tip: Pumps higher than 50 HP, should start thinking about a study

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Reciprocating Pump Piping System Design

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Design Considerations

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Design Considerations

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Pulsation Resonance : Video

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Example: Pulsation Resonance PROBLEM: PSVs popping at line pressures well under PSV set pressure Cost $5,000 every pop to re-certify PSV

PSV

TROUBLESHOOTING 1. PSV OEM recommends new model $$$ 2. Dampener OEM recommends new pulsation dampeners  $$$ 3. Pulsation Design Study ordered, showed pulsation resonance in PSV piping 4. Root cause: Pulsation resonance in PSV line

SOLUTION: Move the location of PSV

Design tip: Pulsation study can help with the troubleshooting process

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Pulsation Dampeners Gas Charged

Maintenance Free

Active/Soft Element

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Reactive/Hard Element

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Pulsation Dampeners Style

Gas Charged

Maintenance Free

Appendage style

Mostly in-line (flow through) • No maintenance • No spare parts • Very reliable, high frequency range

Pros

• Compact • Off the shelf • Generally lower capital cost

Cons

• Generally higher capital • Effective frequency range limited cost • Maintenance required • Often custom design per • Bladder failures remove pulsation application protection • Pulsation amplitude • Gas charging procedure not reductions can be limited always easy

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Pulsation Dampener Sizing Traditional Approach • Empirical calculations • API 674 & OEMs have different empirical sizing methods • Good first step for quoting, but… • Do not account for acoustic resonances in piping system Advanced Approach • Pulsation Design Study • Benefits:    

Empirical Sizing Methods

Pulsation Design Study

Complete range of operating conditions All pump speeds Multiple pump interaction Optimized for:

• • • •

All piping designs Size and Location Type (active vs reactive) Number - sometimes none, one, or two

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Example: Pulsation Dampener PROBLEM: High vibration and pulsation after adding VFD drive (change in pump speed) & change in operating conditions

TROUBLESHOOTING: Pulsation Design Study highlighted inadequate pulsation control with new operating states. Solved the problem for new pumps speeds and operating conditions

Design tip: If VFD added, or flows change by more than 10%, a pulsation study can predict the results, and proactively mitigate risks SOLUTION: Additional Pulsation Controls

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Cavitation Pressure (psi)

Pulsation

Acceleration Head Margin

Mean Pressure Vapor Pressure Time (s)

High pulsation increases the likelihood of cavitation. Acceleration head calcs do not account for pulsation resonances

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Example: Problems Caused by Cavitation BACKGEOUND: • Filtration system added • Head loss calculations done • No pulsation design study PROBLEM: • Cavitation on suction line Charge Pumps

Recip Pumps

Charge Pumps

Original Configuration

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Filtration System New Configuration

Recip Pumps

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Example: Problems Caused by Cavitation SOLUTION: - Change in pipe layout - Speed restriction - Change in filtration process - Up-size the charge pumps for more NPSH Design tip: Perform a pulsation design study before changing pipe layout or adding additional equipment, to avoid pulsation resonances!

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Design Considerations

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Mechanical Considerations • Pulsation design minimizes pulsation-induced forces, but cannot eliminate them completely • Mechanical considerations in – Sizing vessels – Piping layout – Clamping – Small bore attachments

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Mechanical Natural Frequency (MNF) • Frequencies where small forces result in large vibration response of structure • Even small forces at MNF can cause significant amount of vibrations, which can lead to fatigue failures

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Force & MNF in Reciprocating Pumps Most pulsation induced forces occur at

Plunger Passing Frequency (PPF)= RPM X Number of Plungers Forces

MNF

| 1x

Design goal: MNF > 1.2 x PPF

| 2x

Orders of PPF

API 674: Minimum MNF must be greater than 20% above PPF

| 3x

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Example: Clamps Shoe type and rest-only pipe supports good for thermal loads, but cannot restrain dynamic loads of reciprocating pumps

PROBLEM: High Vibrations • •

All clamps must have proper foundation

Calculate the Clamp Spacing to raise MNF above 1.2XPPF- move clamps closer to each other Appropriate type of Clamps should be used, U-Bolt clamps are not recommended

Design tip: Perform a full API 674 mechanical review at the design stage Note: Vibration and thermal pipe stress concerns can both be accommodated.

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Example: Piping Layout PROBLEM: High Vibrations Design tip: Eliminate or reduce elevated piping for reciprocating pumps

• Route piping as close as possible to the foundation, so it can be easily supported • Additional friction and head loss will affect system resistance curve – but recips will easily make up the pressure.

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Example: Small-Bore Attachments (SBA) PROBLEM: Small-Bore Failure • About 70% of leaks are due to SBA failures • Stress concentrations

(threaded connections are Design tip: If a SBA cannot be trouble)

eliminated, shorten the length • Eliminate if possible as much as possible or brace it • Shorten as much as back to the main pipe possible Eliminate • Add braces to main pipe

(not to foundation)

Calgary Pump Symposium 2015

Christmas trees!

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Piping Design for Centrifugal Pump Piping Systems - Summary Design Approach (Centrifugals) • Designed mainly for static and thermal loads • Pipe supports to restrain dynamic loads, generally not required • Hanger and piping shoes are common

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Piping Design for Reciprocating Systems - Summary • Pulsation dampeners and other pulsation control devices generally required • API 674 Pulsation Design Study often necessary • Piping supports should be designed to restrain large dynamic loads • Hold-down type clamps • Clamps at different locations to raise MNF above 1.2X PPF • Piping layout close to foundation • Eliminate or shorten small bore attachments as much as possible

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Reciprocating

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Case Study Calgary Pump Symposium 2015

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Case Study: What’s the problem? BACKGROUND:

1. Field pressure depleting 2. Existing centrifugal pumps highly oversized, high recycle 3. Why not replace centrifugal pumps with a small recip? 4. A triplex diaphragm pump was installed (no pulsation or mechanical study done) 5. Could not be operated due to vibration and pulsation problems!

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Case Study: Video

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Vibration and Pulsation Measurements

11 inch/sec!! Discharge Piping Vibration Vibration Guideline

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Case Study: Pulsation and Mechanical Analyses Mechanical Model

Mechanical Recommendations

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Pulsation Model

Pulsation Recommendations

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Case Study: Recommendation Implementation Many supports were installed to raise MNF of elevated piping

Additional pulsation dampener

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Wrap-Up Calgary Pump Symposium 2015

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Conclusions Reciprocating Pumps generate high pulsation-induced forces in piping. • Therefore, piping design concepts for centrifugal pumps should NOT be applied to reciprocating pumps • Reciprocating Pumps can work well, they just have to be designed well For Reciprocating Pumps: 1. 2. 3. 4. 5. 6. 7. 8.

Pulsation Dampeners are required API 674 Design Study is often necessary (DA1, DA2, or DA3) Pulsations should be controlled and minimized at design stage Piping layout should be as close as possible to foundation for best support Vibration and thermal pipe stress concerns can be both addressed for a good design. Generally piping requires more support (including vibration clamps) Hangers, rest-only supports, and pipe shoes cannot restrain high pulsation induced shaking forces Eliminate, reduce height, or brace small bore attachments

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QUESTIONS? [email protected] [email protected] Calgary Pump Symposium 2015

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Piping Design Considerations for Reciprocating vs Centrifugal Pumps

Piping Design Considerations for Reciprocating vs Centrifugal Pumps Ramin Rahnama, Project Engineer Jordan Grose, Manager, Vibration Integrity Group B...

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