Fluid_Flow_Rules_of_Thumb_for_Chemical

Fluid F l ow

25

2. Type of trim. Use equal percentage whenever there is a large design uncertainty or wide rangeability is desired. Use linear for small uncertainty cases. Limit max/min flow to about 10 for equal per- centage trim and 5 for linear. Equal percentage trim usually requires one larger nominal body size than linear. 3. For good control where possible, make the control valve take 50%-60% of the system flowing head loss. 4. For saturated steam keep control valve outlet veloc- ity below 0.25 mach. The ASME code I provides the basic requirements for over-pressure protection. Section I, Power Boilers, covers fired and unfired steam boilers. All other vessels in- cluding exchanger shells and similar pressure containing equipment fall under Section VIII, Pressure Vessels. API RP 520 and lesser API documents supplement the ASME code. These codes specify allowable accumulation, which is the difference between relieving pressure at which the valve reaches full rated flow and set pressure at which the valve starts to open. Accumulat ion is expressed as per- centage of set pressure in Table 1. The articles by Rearick 2 and Isqacs 3 are used throughout this section. Full liquid containers require protection from thermal expansion. Such relief valves are generally quite small. Two examples are Safety Relief Valves

5. Keep valve inlet velocity below 300 ft/sec for 2" and smaller, and 200ft/sec for larger sizes.

References

1. Fisher Controls Company, Sizing and Selection Data, Catalog 10. 2. Chalfin, Fluor Corp., "Specifying Control Valves," Chemical Engineering, October 14, 1974.

2. Long lines to tank farms that can lie stagnant and exposed to the sun.

Sizing

Use manufacturer 's sizing charts and data where avail- able. In lieu of manufacturer ' s data use the formula

u - 0.442gAh

where

Ah = Head loss in feet of flowing fluid u = Velocity in ft/sec g = 32.2ft/sec 2

1. Cooling water that can be blocked in with hot fluid still flowing on the other side of an exchanger.

This will give a conservative relief valve area. For compressible fluids use Ah corresponding to ~2P~if head difference is greater than that corresponding to ~P1 (since sonic velocity occurs). If head difference is below that corresponding to ~2P~use actual Ala. For vessels filled with only gas or vapor and exposed to fire use

Table 1 Accumulation Expressed as Percentage of Set Pressure

ASME

ASME

Typical Design for Compressors

Section I

Section VIII

Power Boilers

Pressure Vessels

Pumps

0.042As

and Piping

(API RP 520, Reference 4)

A ~ .

LIQUIDS

thermal expansion

- -

10 20

25 20

fire

- -

A = Calculated nozzle area, in. 2 P1 = Set pressure (psig) x (1 + fraction accumulation) + atmospheric pressure, psia. For example, if accu- mulation = 10%, then (1 + fraction accumulat ion)= 1.10

STEAM

over-pressure

3

10 20

10 20

fire

- -

GAS OR VAPOR over-pressure

- -

10 20

10 20

fire

- -

A~ = Exposed surface of vessel, ft2