SYSTECH CONSULTANCY SERVICES

Technical Review of the Effect of Discharge Piping on Pump Performance including Technique to Match Centrifugal Pumps to Piping Systems for its Optimal Operation.

By

D. H. RAWAL, M. E., M. S. (U.S.A.)

___________________________________________________________________________

INTRODUCTION

PUMP

=> HEART

In Refinery and Petrochemical Plants, Pumps are hearts of the fluid conveying system and Effect of Discharge Piping on Pump Performance including Technique to Match Centrifugal Pumps to Piping Systems for its Optimal Operation play an important role.

Effect of Discharge Piping on Pump Performance including Technique to Match Centrifugal Pumps to Piping Systems for its Optimal Operation

We would be covering following two important areas for Effect of Discharge Piping on Pump Performance:

DISCHARGE PIPING - LOCATION OF PUMP WITHIN THE SYSTEM

The pump has to be provided at the optimum location from the point of view of stability of pumping systems. It has been observed for high pressure boiler feed water pumps, the vibrations problems can occur in case pump is not located correctly with reference to discharge and suction piping.

STABILITY OF PUMPING SYSTEMS

In a pumping system, pressure and mass flow fluctuations propagate themselves as acoustic waves. Where these waves are reflected, for example from a closed valve, so-called standing waves are formed with a minimum and maximum for pressure and mass flow fluctuations.

Each characteristic standing wave configuration has its associated natural frequency and damping. In the case, the natural frequencies are given by:

f/4 = C / 4L

f3/4

= 3C / 4L, etc.

= wave length

f/4, f3/4= natural frequencies

C

= velocity of sound in liquid

L

= length of pipeline

Both the information and the damping of standing waves are dependent primarily on the positions of the pumps, valves and other components present in the system. Hence, location of the pump has to be matched.

Acoustic resonance's in a pumping system - arising from a coincidence between an excitation frequencies emanating from a pump, valve or other component and an acoustic natural frequency of the system - have often only a small damping. Thus, pressure pulsations starting out from discharge of the pumps or other components are amplified and lead to non-permissible vibrations of the discharge pipelines.

Hence, it necessary to design in as large as a system damping as possible. Since, however, pipelines in general make only a marginal contribution to the damping of pressure pulsations, this damping has to be achieved via other components, such as throttling sections.

An acoustic model has been formulated which is suitable for analysis of the problem. In this model, each element of the system is respectively by a suitable mathematical model which links pressure and mass flow fluctuations between inlet and outlet.

The objective of the analysis was to develop a model, suitable for practical application, by means of which it is possible to determine the system model of the pump using the pump geometry and the pump characteristic. This is a means of optimizing the acoustic damping of discharge pipeline systems.

A simplified feed water system as an application example. The feed pump model incorporated a slightly unstable characteristic, so that further calculation procedures could take place.

Following the investigation demonstrates that with the valve closed, the natural frequency of the system (13 Hz) is associated with negative damping, i.e. vibrations are excited in the system. In this instance, the second natural frequency corresponds to the 3/4 wave length, which has a mass flow fluctuation q maximum in the region around the pump. Opening of the valve to the half-way position results in all waveforms having positive damping, since the valve now represents a resistance which extracts vibration energy from the system. When the length of pipeline between pump and valve is shortened and the valve closed, the damping becomes positive, since the maximum of the mass flow fluctuation q is displaced away from the pump by this procedure.

The example shows that, by using the model which has been developed, it is possible to design pump systems which are not responsive to exciting pressure pulses emanating from system components. Both the model and the investigations are described.

PUMP PIPING DESIGN - COMPUTER AIDED ENGINEERING ANALYSIS

Computer aided engineering analysis is required during the Effect of Discharge Piping on Pump Performance including Technique to Match Centrifugal Pumps to Piping Systems for its Optimal Operation.

DISCHARGE PIPINGIt must be noted that the piping connecting pump should be properly designed so as to exert minimum loads on the discharge flanges of the pump. Please note that pump is not an anchor point but a machinery casing which can take finite amount of loads. The loads on pump nozzles must meet Clause 2.4 External Nozzle Forces and Moments read along with Table 2 and Appendix F Criteria for Piping Design of API Standard 610.

PIPING SYSTEMS FOR ROTATING EQUIPMENT

For rotary equipment to be functional and contribute to the process system, it must be connected to the system with piping. The science of connecting piping system to rotary equipment is a relatively new field and has drawn the stalwarts of academe to join with industry in solving the problems of piping and equipment.

Discharge Nozzle Loading

In earlier years various rotating equipment manufactures would define allowable nozzle loadings as zero force and zero moments. Such statements were not only ludicrous, but also showed how little confidence some rotary equipment manufactures had in their products. Ultimately, the pipe stress engineer was left to use their sole judgment to determine if the piping loads were substantial enough to damage the attached equipment.

There are several standards for handling nozzle loadings on rotating equipment and probably the best known for pumps from the American Petroleum Institute (API), API Standard 610 Centrifugal Pumps for General Refinery Service.Applying standards to nozzle loadings on rotating equipment leads to argument in which rotating equipment specialists claim that the API standards are only intended for procurement purposes, and the pipe stress engineers, having no other guidelines to follow, assert that the API standards are what is to be practices.

Allowable for inline pumps are available from pump vendors, did not exist a few years ago. Such pumps were regarded as piping components, e.g. valves, and allowable were considered unnecessary. But thinning up casing to reduce material and cost makers such allowables possible, although controversial at times.

There are three possible option to solve nozzle loadings on rotating equipment.

1. A detailed finite element (FEM) study of equipment.

2. Destructive testing of the equipment.

3. Close interface between the rotating equipment manufacturer and the piping stress engineer.

The problem with finite element analyses is who is going to pay for it- the client, the engineering contractor, or the rotating equipment manufacturer? Next, can the rotating equipment manufacturer disclose proprietary information often-required in finite element analyses? Destructive testing poses the same question, who will pay for it? The third option-the pipe stress engineer conferring with the equipment manufacturer-is perhaps the most viable of three, because if the API criteria cannot be met, then the rotating equipment manufacturer can least expect extra loading and can design for it, if time permits. Thus, the rotary equipment vendor working as a team with the piping stress engineer(s) can help to alleviate most nozzles loading problems.

API standards are very safe and a piece of equipment that meets their requirements should not have any nozzle loading problems, such as leaks. The problem comes in modular skid construction, where the values provided by the standards are very conservative.

Manufacturers often give allowable values for their equipment, which can be used.

CONCLUSIONS

Hence, this technical review shows the Effect of Discharge Piping on Pump Performance including Technique to Match Centrifugal Pumps to Piping Systems for its Optimal Operation.

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