Example 3-19 Pump Specifications

The pump specified identifies the design data, key portions of the construction materials and driver data as required information for the pump manufacturer. If the pump is to be inquiried to several manufacturers this is all that is necessary. The individual manufacturers will identity dieir particular pump selection and details of construction materials and driver data. From this information a pump can be selected with performance, materials of construction, and driver requirements specified. 

In the example the manufacturer has been specified from available performance curves, and the details of construction must be obtained. The pump is selected to operate at 22 GPM and 196 to 200 feet head of fluid, and must also perform at good efficiency at 18 GPM and a head which has not been calculated, but which will be close to 196 lo 200 feet, say about 185 feet. Ordinarily, the pump is rated as shown on the specification sheet. This insures adequate capacity and head at conditions somewhat in excess of normal. In this case the design GPM w as determined by adding 10 percent to the capacity and allowing for operation at 90 percent of the rated efficiency. Often this latter condition is not considered, although factors of safety of 20 percent are not unusual. However, the efficiency must be noted and the increase in horsepower recognized as factors w hich are mounted onto normal operating conditions. 

Sometimes the speed of the pump is specified by the purchaser. However, this should not be done unless there is experience to indicate the value of this, such as packing life, corrosion/erosion at high speeds, and suspended particles as the limitation oil speed may prevent the manufacturer from selecting a smaller pump. In some cases it must be recognized that high heads cannot be reached at low speeds in single stage pumps. Table 3-7 presents sug- 

Applied Process Design for Chemical and Petrochemical Plants 

RotImbmp 14.6 on-Overload BHP 15 Shut-afl Rraf. 45 P9lg— NPSH Ruauirad f  

---

The system is optimized. Steps 1-12 must be repeated for each candidate pipe. The entire set must then be cost optimized. For example, the design indicated by Fig. 8 will work hydraulically but is not optimum. We see that at mp-60, the interior control point, we have actually cleared GP by 342 ft. This is considerably more than the minimum 50-ft terrain clearance required and is therefore wasteful of pumpinig energy. The design can obviously be improved by a change in pump specifications and other details. This should be done for each candidate pipe. The final design to be selected is based on an economic minimum-cost evaluation. 

A SPECIFIC EXAMPLE PUMPING STATION SCHUWACHT, HYDRON-ZH... 

Prior to cellular excitation, an electrical gradient exists between the inside and the outside of the cell membrane. At this time, the cell is polarized. In atrial and ventricular conducting tissue, the intracellular space is about 80 to 90 mV negative with respect to the extracellular environment. The electrical gradient just prior to excitation is referred to as resting membrane potential (RMP) and is the result of differences in ion concentrations between the inside and the outside of the cell. At RMP, the cell is polarized primarily by the action of active membrane ion pumps, the most notable of these being the sodium-potassium pump. For example, this specific pump (in addition to other systems) attempts to maintain the intracellular sodium concentration at 5-15 mEq/L and the extracellular sodium concentration at 135-142 mEq/L and the intracellular potassium concentration at 135-140 mEq/L and the extracellular potassium concentration at 3-5 mEq/L. RMP can be calculated by using the Nemst equation ... 

The terms failure and fault have specific meanings in the context of risk management. Failure refers to the nonfunctioning of a specific item of equipment fault refers to the nonfunctioning of a system or subsystem. For example, Pump P-IOIA may fail to operate. If the backup pump P-IOIB does not start, then a fault exists with the pumping system. (In practice, it is unusual for this semantic distinction to be scrupulously followed.)... 

Plants that build redundancy nlo their processes provide backup systems for critical equipment. For example, pumps and compressors typically have two or three backups. Process technicians should attempt to uncover as much information as possible about the equipment used in their units. Much of this information can be found in technical manuals or the operating manuals. Manufacturer information is typically included in the engineering specifications, drawings, and equipment descriptions. 

For example, the specification of admissible stress for a swimming pool pump takes the following form ... 

Process Unit or Batch Unit A process unit is a collection of processing equipment that can, at least at certain times, be operated in a manner completely independent from the remainder of the plant. A process unit normally provides a specific function in the production of a batch of product . For example, a process unit might be a reactor complete with all associated equipment (jacket, recirculation pump, reflux condenser, and so on). However, each feed preparation tank is usually a separate process unit. With this separation, preparation of the feed for the next batch can be started as soon as the feed tank is emptied for the current batch. 

Item of Equipment An item of equipment is a hardware item that performs a specific purpose. Examples are pumps, heat exchangers, agitators, and the like. A process unit could consist of a single item of equipment, but most process units consist of several items of equipment that must be operated in harmony in order to achieve the function expec ted of the process unit. 

The cost of the filter station includes not only the installed cost of the filter itself but also that of all the accessories dedicated to the filtration operation. Examples are feed pumps and storage facihties, precoat tanks, vacuum systems (often a major cost factor for a vacuum filter station), and compressed-air systems. The dehvered cost of the accessories plus the cost of installation of filter and accessories generally is of the same order of magnitude as the dehvered filter cost and commonly is several times as large. Installation costs, of course, must be estimated with reference to local labor costs and site-specific considerations. 

Equipment Constraints These are the physical constraints for individual pieces of eqiiipment within a unit. Examples of these are flooding and weeping limits in distillation towers, specific pump curves, neat exchanger areas and configurations, and reactor volume limits. Equipment constraints may be imposed when the operation of two pieces of equipment within the unit work together to maintain safety, efficiency, or quahty. An example of this is the temperature constraint imposed on reactors beyond which heat removal is less than heat generation, leading to the potential of a runaway. While this temperature could be interpreted as a process constraint, it is due to the equipment limitations that the temperature is set. 

Some pump companies will promote and tout their low Nss values. Sometimes a specification engineer will establish a maximum Nss limit for quoted pumps. Let s consider the.se examples of operating parameters of pumps, and determine the Nss. These values are lifted from the pump performance curves at the BLP. 

The majority of centrifugal pumps have performance curves with the aforementioned profiles. Of course, special design pumps have curves with variations. Eor example, positive displacement pumps, multi-stage pumps, regenerative turbine type pumps, and pumps with a high specific speed (Ns) fall outside the norm. But you ll find that the standard pump curve profiles are applicable to about 95% of all pumps in the majority of industrial plants. The important thing is to become familiar with pump curves and know how to interpret the information. 

A different set of forms, in extensive use for failure rate calculation, are used to illustrate the remaining sections of this chapter. Beginning with Figure 6.3, the forms present a worked pump example for the conversion of actual plant raw data to plant-specific failure rate data. 

Knowing the serial number allows detailed information on a machine or component to be obtained. This is possible because machinery manufacturers must maintain records for their products. These records, which are usually identified by serial number, contain complete design and performance data for that specific unit. For example, it is possible to obtain a performance curve or complete bill of materials for each pump found in a plant. 

Figure 51.3 provides an example of a typical work breakdown stmcture. The major divisions define specific, logical task groupings as well as the cost-accounting classifications. In the example, cost should be acquired for the preparation of the foundation, installation and final com-misioning of the pump and motor. In addition, each of the sub-tasks that comprise these classifications are logical groups of tasks that must be completed in sequence and by different work classifications. 

---