Valves equivalent length

The flow resistance of pipe fittings (elbows, tees, etc) and valves is expressed in terms of either an equivalent length of straight pipe or velocity head loss (head loss = Kv /2g ). Most handbooks and manufacturers pubHcations dealing with fluid flow incorporate either tables of equivalent lengths for fittings and valves or K values for velocity head loss. Inasmuch as the velocity in the equipment is generally much lower than in the pipe, a pressure loss equal to at least one velocity head occurs when the fluid is accelerated to the pipe velocity. 

Although it has been common practice to specify the pressure loss in ordinary valves in terms of either equivalent length of straight pipe of the same size or velocity head loss, it is becoming more common to specify flow rate and pressure drop characteristics in the same terms as has been the practice for valves designed specifically for control service, namely, in terms of the valve coefficient, C. The flow coefficient of a valve is defined as the volume of Hquid at a specified density that flows through the fully opened valve with a unit pressure drop, eg, = 1 when 3.79 L/min (1 gal /min) pass through the valve... 

The equivalent length of the main flare header is then calculated from the flare stack to the last safety valve, taking into consideration the straight length of the pipe and approximate equivalent lengths for bends, etc. If the achial location of the flare stack is not known by that time, it maybe assumed to be 500 ft from the last piece of equipment. Later on, even if it varies from 500 ft, it will not affect the pressure drop calculation at all compared with the entire length of the pipe. 

From Figure 2-20 or 2-21 determine the equivalent lengths of all fittings, valves, etc. 

Tabulate the summation of equivalent lengths of straight pipe, valves, fittings, entrance/exit losses as presented in earlier sections of this chapter. 

L(,q = Equivalent length of pipe plus equivalent length of fittings, valves, etc., ft. 

Representative Equivalent Length in Pipe Diameters (L/D) of Various Valves and Fittings [1]... 

For pipe fittings, valves, and other flow obstructions the traditional method has been to use an equivalent pipe length Lequiv in Equation 4-30. The problem with this method is that the specified length is coupled to the friction factor. An improved approach is to use the 2-K method,s-6 which uses the actual flow path length in Equation 4-30 — equivalent lengths are not used — and provides a more detailed approach for pipe fittings, inlets, and outlets. The 2-K method defines the excess head loss in terms of two constants, the Reynolds number and the pipe internal diameter ... 

Equivalent lengths of various valves and fittings are readily available [Holland and Chapman (1966)] and a selection is given in Table 2.3. 

Correlations are available for pressure drops in flowthrough pipe fittings, such as elbows, bends, and valves, and for sudden contractions and enlargements of the pipe diameter as the ratio of equivalent length of straight pipe to its diameter. 

Friction due to fittings, valves and other disturbances of flow in pipe lines is accounted for by the concepts of either their equivalent lengths of pipe or multiples of the velocity head. Accordingly, the pressure drop equation assumes either of the forms... 

The value K = 0.05 for gate valve from Table 6.5 appears to be low Chemical Engineering Handbook, for example, gives 0.17, more nearly in line with that from Table 6.6. The equivalent length method of Table 6.4 gives high pressure drops although convenient, it is not widely used. 

A centrifugal pump and its spare handle 1000 gpm of an oil at 500°F. Its specific gravity is 0.81 and its viscosity is 3.0 cP. The length of the line is 600 ft and its equivalent length with valves and other fittings is 900 ft. There are 12 gate valves, two check valves, and one control valve. 

The pressure loss through the bends and check valves can be included in the line pressure-loss calculations as an equivalent length of pipe . The equivalent-length recommendations are taken from Ref. P1 (p.164). All the bends are assumed to be 90 elbows of standard radius, and the isolation valves as plug-type valves. 

FIGURE 6.5 Equivalent length of pipe fittings and valves. (Crane Co.)... 

The equivalent length of one 8-in-long-radius elbow is 14 ft of pipe, from Table 6.22. Since the pipe contains four elbows, the total equivalent length is 4(14) = 56 ft of straight pipe. The open gate valve has an equivalent resistance of 4.5 ft, and the open swing check valve has an equivalent resistance of 53 ft. 

---