Engineering Of Pumping System Requirements

The first step is to define the requirements and conditions under which the equipment will operate.

Fluid Type A thorough description of the fluid to be handled must be developed. This includes properties such as viscosity, density, vapor pressure, corrosiveness, erosiveness, volatility, flammability, and toxicity. Depending on the process and the system, some or all of these properties may have an important effect on the pump and system design. For example

• The corrosiveness of the fluid will influence the materials of construction.

• If the fluid contains solids in suspension, suitable types of pump seal designs and abrasion-resistant pump construction must be considered.

• Erosion due to high particle content may cause premature performance decline. Large particles may favor open impeller design.

• Fluid toxicity may necessitate the use of dual (tandem or double) mechanical seals due to government regulations or safety considerations.

• Entrained gases may affect the pump's ability to produce the required differential pressure.

The specified fluid physical and chemical properties must cover the entire expected operating range of the pumping system. Influences such as varying temperatures and pressures must also be defined.

System Head Curves The engineer must have a clear understanding of the process and system in which the pumping equipment will operate. A preliminary design of the system should be made and should include an equipment layout and a P&ID (piping and instrument diagram). These preliminary drawings will show the various fluid flow paths for system operation, preliminary pipe diameters and lengths, relative elevations of system components and all valves and other piping components that will be used to establish the system head losses. These drawings will be used by the engineer to calculate the final piping sizes and pumping system head requirements.

With this information, the engineer can develop system head curves that show the relationship between flow rate and hydraulic losses in the piping system. In determining the hydraulic losses, the engineer must include adequate allowances for future corrosion and scale deposits in the piping system over the plant life.

Because hydraulic losses are a function of flow rate, pipe size, and layout, each individual flow path alignment in a given system will have its own characteristic operating curve. Care must be taken when specifying the required pump characteristics to take into account all possible system operating flow paths. It is convenient to add the effects of static pressure and elevation differences in the system to form a combined system head curve. This combined curve shows the total head required of the pumping equipment to overcome system resistance as well as differential static pressure and elevation. The pump head must be at or above the combined system curve at all required operating points and fluid conditions for the various system flow paths. Refer to Sections 8.1, 8.2, 9.1, and 9.2 for guidance in constructing system head curves.

Modes of System Operation System operating modes are important considerations when specifying pumping equipment. Will the pump be used in continuous or intermittent operation? Will the pump operate in parallel or series with other pumps? Will there be significant differences in head or flow rate requirements in different system alignments? Will a single pump be used as a common spare for two different pumping applications? These and other questions arising from analyzing the different modes of operation will help influence decisions as to the number of pumps needed, heads and capacities and whether booster pumps are desirable in some system alignments. It should be noted that unnecessarily conservative hydraulic requirements may increase pump complexity (such as the selection of a more elaborate multistage or double suction pump in place of simpler single stage, overhung pump) and cost.

The engineer should also consider the length of time between plant maintenance expected of the pumping system. This factor will influence the decision of quantity, pump type, requirement for installed spare(s), and the manufacturing quality required of the specified pumps. Frequently, due to the critical nature of a pumping service where high reliability is necessary, installed spares are provided. In some cases, 2-100% pumps are provided. When system flowrate requirements fluctuate, 3-50% pumps may be called for. When reduced flowrates will not adversely affect operations, 2-50% pumps can be specified. Plant operating philosophies will dictate if automatic start of a spare pump is required.

Pump Flow/Head Margins Pumps are normally specified with a capacity margin above what has been determined necessary for the process. In addition, the calculated system head losses are also determined conservatively. The reasons for this include the following:

• During system design, many assumptions are made while determining pump requirements, some of which might eventually be determined to be incorrect.

• During the plant life cycle, process conditions are likely to change due to aging catalyst, changes in feed stock, seasonal feed temperature variations, and so on.

• Final piping design may be significantly different from preliminary design.

• System hydraulic losses may change due to corrosion, and so on.

During preliminary system design, these potential future changes in head/capacity must be studied to determine the required design margin. Because a pump should be selected to operate close to its best efficiency point, it is important to minimize the selected margin. Margins of 5 to 10% on flow are typical but even 20% is common, for example, in reflux tower service. In cases where the process is well proven and understood, and system operating requirements are well defined, a zero margin is sometimes appropriate.

Plans for future capacity increase may be foiled if the piping system does not provide for adequate net positive suction head at the future flow conditions.

Care must be taken when applying margins to ensure that the purchased pump is not oversized. If the total head produced is too large, impellers may be trimmed within the allowable range of the pump model provided. If the flow is significantly oversized, a costly energy penalty can result over the life of the plant. This is caused by lower pump operating efficiency at off-design flow rates and discharge throttling losses that may be necessary to control the flow rate to the desired value.

Type of Pump Control The type of control for the required pump is also an important consideration for pump specification and selection. Since the actual piping system usually incorporates a design margin, a control valve is normally employed in centrifugal pump applications. This control valve (not supplied by the pump manufacturer) is used to adjust the system curve over the life of the unit. Flow sensing control provides the most stable operation for most systems. Pressure control can have very large swings in flow when operating in a flat or drooping area of a centrifugal pump curve. For this reason, most centrifugal pump specifications incorporate the requirement for the centrifugal pump operating curve to rise continually to pump discharge valve shut in (also known as the pump shut off).

Both temperature and level sensing control can lead to a pump running at shut off or at the end-of-curve condition during upset or failure modes. Centrifugal pumps with a 10% to 20% head rise between the specified operating point and the shut off point may be preferred for some services such as for parallel pump operation.

Pumps that may operate in the shut off condition may need a continuously open bypass to prevent pump damage. Pump manufacturers will advise of the minimum required flow for an offered pump, but many end users require an additional margin above the manufacturers recommended minimum flow. If a continuous flow bypass is incorporated in the system design, this additional flow must be added to the specified pump requirement. Refer to Section 2.3.4 for bypass designs.

Future System Changes A final factor to be considered is the possibility of providing for future system changes. When future system changes can be predicted with a degree of certainty, the system can be designed with this in mind. Rather than selecting a pump that is operating at the high end of its preferred operating region, a larger impeller diameter or the next larger size pump operating at the beginning of its preferred operating range might be considered. In addition, the capability for installing a larger diameter impeller to handle future higher head requirements must be considered. Because minimizing capital costs for a project is usually the prime consideration, oversizing a pump for future operations is not normal practice. Pumps are expected to operate efficiently and reliably in the present system, and this fact should be noted during the selection progress.

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