Fluid_Flow_Rules_of_Thumb_for_Chemical

Fluid Flow

27

Liquid Expansion

The environmental factors represented by F are

Bare v e s s e l - 1.0 I nsu l a t ed - 0.3/insulation thickness, in. Underground storage - 0.0 Earth covered above grade - 0.03

The following equation can be used for sizing relief valves for liquid expansion.

BH 500GC

Q m

(API RP 520, Reference 4)

The height above grade for calculating wetted surface should be

where

1. For vertical ve s s e l s~a t least 25 feet above grade or other level at which a fire could be sustained. 2. For horizontal ve s s e l s~a t least equal to the maximum diameter. 3. For spheres or spheroids~whi chever is greater, the equator or 25 feet. Three cases exist for vessels exposed to fire as pointed out by Wong. 6 A gas filled vessel, below 25 ft (flame heights usually stay below this), cannot be protected by a PSV alone. The metal wall will overheat long before the pressure reaches the PSV set point. Wong discusses a number of protective measures. A vessel containing a high boiling point liquid is similar because very little vapor is formed at the relieving pressure, so there is very little heat of vaporization to soak up the fire's heat input. A low-boiling-point liquid, in boiling off, has a good heat transfer coefficient to help cool the wall and buy time. Calculate the time required to heat up the liquid and vaporize the inventory. If the time is less than 15 minutes treat the vessel as being gas filled. If the time is more than 15-20 minutes treat it as a safe condition. However, in this event, be sure to check the final pressure of the vessel with the last drop of liquid for PSV sizing.

Q - Required capacity, gpm H - Heat input, Btu/hr B -Coef f i c i ent of volumetric expansion per ~ = 0.0001 for water = 0.0010 for light hydrocarbons

= 0.0008 for gasoline = 0.0006 for distillates = 0.0004 for residual fuel oil G - Specific gravity C - Specific heat, Btu/lb ~

Rules of Thumb for Safety Relief Valves

1. Check metallurgy for light hydrocarbons flash- ing during relief. Very low temperatures can be produced. 2. Always check for reaction force from the tailpipe. 3. Hand jacks are a big help on large relief valves for several reasons. One is to give the operator a chance to reseat a leaking relief valve. 4. Flat seated valves have an advantage over bevel seated valves if the plant forces have to reface the surfaces (usually happens at midnight). 5. The maximum pressure from an explosion of a hydrocarbon and air is 7 x initial pressure, unless it occurs in a long pipe where a standing wave can be set up. It may be cheaper to design some small vessels to withstand an explosion than to provide a safety relief system. It is typical to specify ~" as minimum plate thickness (for carbon steel only).

Heat Exchanger Tube Failure

1. Use the fluid entering from twice the cross section of one tube as stated in API RP 5204 (one tube cut in half exposes two cross sections at the cut). 4 2. Use Ah - uZ/2g to calculate leakage. Since this acts similar to an orifice, we need a coefficient; use 0.7. So,

Sources

1. ASME Boiler and Pressure Vessel Code, Sections I and VIII. 2. Rearick, "How to Design Pressure Relief Systems," Parts I and II, Hydrocarbon Processing, August/ September 1969.

u - 0.%/2gAh

For compressible fluids, if the downstream head is less than ~ the upstream head, use ~2the upstream head as Ah. Otherwise use the actual Ah.