Required Head

A typical piping application starts with a specified flow rate for a given fluid. The piping system is then designed with the necessary valves, fittings, etc. and should be sized for the most economical pipe size, as discussed in Chapter 7. Application of the energy balance (Bernoulli) equation to the entire system, from the upstream end (point 1) to the downstream end (point 2) determines the overall net driving force (DF) in the system required to overcome the frictional resistance  

The total head (driving force) is the net sum of the pump head, the total pressure drop, and the elevation drop  

The friction loss (J] ef) is the sum of all of the losses from point 1 (upstream) to point 2 (downstream)  

This relates the system pump head requirement to the specified flow rate and the system loss parameters (e.g., the K values). Note that Hp is a quadratic 

---

First, the required head is calculated. Either the polytropic or adiabatic head can be used to calculate horsepower so long as the polytropic or adiabatic efficiency is used with the companion head. 

Calculate the overall required head from the pressure ratio and the inlet temperature using Equation 2.70 from Chapter 2. It is repeated here for convenience. 

Pumps are operated in parallel to divide the load between two (or more) smaller pumps rather than a single large one, or to provide additional capacity in a system on short nodce, or for many other related reasons. Figure 3-35 illustrates the operational curve of two identical pumps in parallel, each pump handling one half the capacity at the system head conditions. In the parallel arrangement of two or more pumps of the same or different characteristic curves, the capacities of each pump are added, at the head of the system, to obtain the delivery flow of the pump system. Each pump does not have to carry the same How but it will operate on its own characteristic curve, and must deliver the required head. At a common tie point on the discharge of all the pumps, the head will be the same for each pump, regardless of its flow. 

Operations such as blending, solids-suspension, dissolving, heat transfer and liquid-liquid extraction are typical of systems requiring high flow relative to turbulence, while gas-liquid reactions and some liquid-liquid contacting require high turbulence relative to flow. The case of (1) 100% of suspension—requires head to keep particles suspended and (2) 100% uniformity of distribution of particles—requires head for suspension plus flow for dis-tiibution. 

Most pump manufacturers provide composite curves, such as those shown in Fig. 8-3, that show the operating range of various pumps. For each pump that provides the required flow rate and head, the individual pump characteristics (such as those shown in Fig. 8-2 and Appendix H) are then consulted. The intersection of the system curve with the pump characteristic curve for a given impeller determines the pump operating point. The impeller diameter is selected that will produce the required head (or greater at the specified flow rate). This is repeated for all possible pump, impeller, and speed combinations to determine the combination that results in the highest efficiency (i.e., least power requirement). Note that if the operating point (Hp, Q) does not fall exactly on one of the (impeller) curves, then the... 

Blast loaded structures produce high reaction loads at column supports. This usually requires substantial base plates as well as high capacity anchor bolts. Achieving full anchorage of these bolts is of primary importance and will usually require headed bolts or plates at the embedded end of the bolts to prevent pullout. When anchor bolts are securely anchored into concrete, the failure mechanism is a ductile, tensile failure of the bolt steel. Insufficient edge distance or insufficient spacing between bolts results in a lower anchorage capacity and a brittle failure mode. 

The required head loss (pressure drop), across the control valve, Ahcv, at any desired operating point (op point) can easily be determined, see Figure 15. An operating range of head losses required across a control valve, Ahcv min to Ahcv max, say, may also be determined. 

To use Fig. 6.32, enter at the bottom at the required capacity, 750 gal/min, and project vertically to intersect the 100-ft head curve, the required head. From here project horizontally to the 1000-SSU viscosity curve and then vertically upward to the correction-factor curves. Read CE = 0.635 Cq = 0.95 CH = 0.92 for 1.0QNW. The subscripts E, Q, and H refer to correction factors for efficiency, capacity, and head, respectively, and NW refers to the water capacity at a particular efficiency. At maximum efficiency, the water capacity is given as 1.0 Qmw other efficiencies, expressed by numbers equal to or less than unity, give different capacities. 

For purposes of example, assume a flow of 8.71 mVmin (2300 gal/min) through the tower The maximum head available to the recovery turbine was calculated to be 604 m (1982 ft) this value will be slightly in error when part of the flow is bypassed since frictional losses into and out of the recovery unit will change. First, assume the lean pump to be at 3.03 mVmin (800 gahmin) running at 3900 r/min with the semilean pump at 5.68 mVmin (1500 gal/min) to get the total flow of 8.71 mVmin (2300 gal/min). At 3.03 mVmin (800 gal/min) and 3900 r/min the available head of the lean pump is read from the curve. This must be greater than the required head, and the excess is plotted as in Fig. 29-60. The brake horsepower of the lean pump is also read. 

The ASME design formula from UG-34 for circular flat heads subjected to internal pressure is t = d CP/SEf, where t is the minimum thickness of the head (in.), d is the internal diameter of the vessel (in.), P is the MAWP (psi), S is the allowable stress in the material listed in ASME Section II, and E is the weld joint efficiency. The value of C ranges from 0.10 to 0.75 depending on the method of attachment of the head and the shell dimensions. For preliminary designs, a value of 0.33 for C will lead to a good approximation of the required head thickness. 

It would be useful to sort grades according to price, so the cheapest grade that meets the requirements heads the list. However, the prices of plastics fluctuate with the price of oil (Chapter 2) and CAMPUS does not give prices. It indicates whether a grade is suitable for a particular process, but the database neither indicates the cheapest process route, nor gives the cost of the manufactured product. 

Tlie required head thickness shall be the greater of that required for external pressure or that required for an internal pressure equal to 1,67 x P. See Reference 1, Para. UG-33(a). 

---