SAFELY SIZE AND DESIGN
It is often desirable to combine the discharges from safety relief valves
into common pipe headers. The common headers are piped to a safe location, with
provision for collecting liquid relief and treating vapor discharge.
VentManifold is an Excel spreadsheet template that calculates the
backpressure that develops when simultaneous vents discharge into a common
header. This is important because relief valves are designed to operate within
specified back pressure limits.
This page discusses the design of relief valve discharge manifolds.
A relief valve discharge header takes the form of a tree. Each valve, vent,
or rupture disc is piped to a branch; branches may combine into larger branches.
Finally, the main trunk is reached, discharging to atmosphere (perhaps by way of
a large knock-out drum or scrubber).
After the equipment arrangement is established, a rough pipe routing is made.
Then the piping may be sized using the methods presented here. A detailed
routing is designed, being sure that there are no pockets where liquid could
obstruct the flow. Sizing should be checked with the detailed routing, again
using the procedures presented here.
Relief manifold pipe sizing is critical to the reliable operation of the
system. The discharge piping produces back pressure on the relief valves. Larger
diameter piping results in lower back pressure. Relief valves are designed to
work with back pressure from 10% (conventional valves) to 50% (balanced valves)
of their set pressure. For example, a conventional valve relieving at 100 psig
will work reliably if the back pressure does not exceed 10 psig.
When determining the allowable back pressure, manufacturer's test data should
be used for the specific valve in question. Do not rely on "rules of
thumb." Balanced valves will relieve at their rated set point, even with
high back pressure (up to 80% of set pressure). However, at high back pressure,
the valve capacity (lb per hr of flowing material) is derated. So if a balanced
valve is installed that is exactly matched to the service, it may be that the
back pressure is limited to about 35 to 40% of set pressure.
For example, consider a Farris Balanseal valve with a 100 psig set point. The
valve is rated at 100% of its nominal capacity up to 35% back pressure. At
higher back pressures, the capacity is reduced. At 60% back pressure (i.e.,
60 psig), the valve is derated to 88% of its normal capacity.
The possibility of simultaneous discharge from multiple valves makes line
sizing difficult. For valves sized for protecting tanks against external fires,
NFPA-30 requires that it be assumed all vessels connected to the manifold will
relieve at once. Installations where the relieving conditions are based on other
criteria, unchecked exothermic reactions for instance, should be analyzed to
determine how many devices could conceivably relieve simultaneously.
When it has been determined which valves will relieve simultaneously, the
task of pipe sizing can commence. Each segment in the manifold network is
analyzed using the flow rates summed from upstream relief valves. It may be
necessary to perform the calculations under multiple scenarios.
Temperature is an important vapor-phase property; it affects the gas density
and viscosity with direct impact on pressure drop. Each pipe segment can
potentially convey a different mix of materials, depending on the contents of
the vessels relieving into the manifold. To determine the temperature for the
system, I advise using the relieving temperature at each relief valve as the
starting point. Assume isothermal flow throughout the system. Where sub-headers
join, compute the mixture temperature (and viscosity) using the mole fraction
mixing rule. If the components are likely to react with
each other in the header those vessels have no business being piped together!
The best way to size the piping is to work backwards (upstream) from the
point where the manifold discharges to atmosphere or a treatment unit. The
general approach is to make a guess for pipe sizes. Then a detailed computation
is performed to determine the pressure drop through each segment. The back
pressure calculated at each relief valve is compared with the allowable back
pressure for the valve. Judgment is used to adjust pipe sizes in the network
(larger or smaller); the calculations are done again and the procedure continued
until back pressures are all within tolerance.
Bear in mind that the minimum allowable pipe size is the size of the
discharge flange on the relief valves. Also, critical (sonic) velocity can not
be exceeded in a pipe.
SHOULD WE SIZE FOR TWO-PHASE FLOW?
Two-phase flow through safety relief valves is expected. The Design Institute
for Emergency Relief System (DIERS) method is used to predict the quantity and
quality of two-phase flow relieved from a reactor (Ref 1). A careful
experimental program, extensive data collection and sophisticated computer
simulation (SAFIRE) are required to adequately analyze this complex situation (Ref
The scope of this article is limited to all vapor flow.
It is applicable when it is known that only vapor will be relieved, or when the
liquid portion is assumed to flash. Where mixed flow is present, and the total
mass quantity (flow rate) is known, an all vapor model will yield conservative
results. It may be prudent to be conservative given the uncertainty of two-phase
Although the piping may be sized for all vapor flow, liquid in the vent line
should not be ignored. The piping design must account for its possible presence.
Piping should include drains at low points, sloping to drain, knock-out pots and
the practice of connecting each subheader into the top of a downstream header.
The amount of liquid may be considerable. It is not unusual for the entire
contents of a reactor to discharge into the vent header; careful analyses of
causes and consequences of emergency situations are required to properly size
and design the liquid handling provisions.
Vertical pipes create a special problem. Two-phase flow is influenced by
gravity. The discharge pressure determines how high above the relief device the
pipe may go; at low discharge pressures (e.g., 10 psig), the gravity
effect will predominate and severely limit the vertical distance that can be
achieved. In severe cases, there may be no flow at all due to the liquid
component collapsing under its weight.
To obtain the minimum total cost, a number of scenarios may be studied.
Consider the effect of different failure assumptions. As with any piping system,
the route chosen for running the pipe can have a major impact on cost.
Comparisons should be made between running one manifold with the cost of
dividing the header into two or more systems. Multiple manifolds may result in
greater total length of pipe, but much of it will be smaller diameter than that
required by a single header system. This is especially true when there is a wide
range of relieving pressures. If a single manifold is sized, the lowest relief
pressure will often dictate the back pressure; removing the low pressure valves
from the manifold may result in much smaller pipe sizes due to the higher back
pressure that can be tolerated.
The sizing calculations should be continued until each relief valve is
connected to the minimum allowable pipe size (i.e., the size of the
outlet flange) or is presented with the maximum permissible back pressure. To
minimize pipe sizes, especially when the runs are long, balanced valves should
be considered instead of conventional type. This won't always result in savings:
the minimum pipe size may be dictated by the size of the valve outlet flange or
the critical velocity of the fluid.
Balanced valves typically cost less than 50% more than conventional type. It
may take less than 50 feet of pipe, reduced by one size (e.g., 6 inch to
4 inch), to economically justify the higher cost valve.
This section presents some basic guidelines to follow when the header piping
is being designed and installed. Manifolds are treated similarly to other
process piping: The ANSI Pressure Piping Code (B31) should be followed.
Important factors in the system design are:
• Avoid Obstructions
Check existing headers for obstructions, valves, pluggages, etc.
Provide clean-out ports or connections. This will enable future inspection
and maintenance of the lines. After installation, use the inspection
• Provide Adequate Supports
Manifold piping should be independently supported from the relief valve,
and carefully aligned to avoid mechanical stress. Reaction forces from valve
discharges and at pipe segment intersections must be considered. Consider
thermal stresses in the manifold, originating from environmental sources
(radiation from sun, adjacent operating process equipment) or from the
relief itself. Reduce the effect of discharge forces by using "Y"
connections in lieu of "T"s.
• Test per ANSI B31
Hydrostatically test piping to 150% of the maximum anticipated pressure
of the system, or pneumatically tested to 110% of the maximum anticipated
pressure. Remove relief valves from piping prior to performing the pressure
• Discharge Safely
The manifold must discharge to a safe location. Where condensable or
toxic materials are present, some type of collection or treatment operation
is required. Examples are water scrubbers and flares. Class I materials
(flash point < 100°F) must be discharged vertically or horizontally at
least 12 feet above the adjacent ground level and at least 5 feet away from
building openings. Corrosive or toxic vapors may need chemical
neutralization (in a scrubber) prior to release. Atmospheric discharge
should be limited to vapors that will not condense at the lowest
temperatures encountered in that locality.
Reference 3 provides detailed recommendations for the
design of catch tanks, scrubbers and flares. Two or three phase flow is
expected and must be considered when designing containment and treatment
equipment. Cyclone type separators are more effective than traditional style
• Eliminate Contamination by Foreign Matter
Provisions are needed to prevent the entrance (and accumulation) of
rainwater into the manifold. Rain caps are acceptable for the purpose; their
effect on back pressure must be included in the sizing calculations. Also
consider that many vent manifolds will rarely see actual service; birds or
rodents may find the empty piping an ideal place to nest so guard against
this by incorporating an appropriate barrier.
• Design Piping to be Self-Draining
The discharge system should drain toward the discharge end, avoiding
pockets if possible. Unavoidable pockets should be fitted with drip legs or
knock-out pots. Piping sloped at 1/4 inch per foot is preferred. Branches
enter trunks from the top.
PROCEDURE FOR PIPE SIZING
Detailed instructions are provided for sizing a relief manifold. A computer
spreadsheet program is useful for carrying out the computations; the examples
show the steps in creating a spreadsheet such as
VentManifold, available from
chemengsoftware.com. Formulas are presented on a
The procedure first requires that a system sketch be prepared. Basic data for
flow rates and physical properties are collected. A material balance for the
manifold is made.
Here are details.
1. System Sketch
Prepare a sketch of the manifold system. It shows the actual or proposed
piping configuration. Each pipe intersection ("node") is labeled.
For clarity, show the equipment being relieved.
2. Basic Data
Make a table with basic data.
from the VentManifold Data Input area. Each of the pipe segments is listed.
It helps to list them in a sequence that begins with the segment discharging
to atmosphere (or the treatment device), then working back through the
manifold, finally listing the segments that connect directly to the relief
valves. List the Node labels for the upstream and downstream end of each
Actual or proposed pipe diameters are entered. The example uses nominal
pipe dimensions, but actual inside diameters will make the calculations more
The equivalent length for each segment is entered. To get this, you need
to know how the pipe is (or will be) routed in the plant. Count or estimate
the number of elbows, tees and other fittings. Measure or estimate the
physical length of the segments. Compute the equivalent length by adding the
physical length to the equivalent length of fittings.
For each relief valve, enter the material name, mass flow rate, molecular
weight, temperature, viscosity and allowable back pressure. These values
must all be at the relieving conditions. In the
the methanol vessels are assumed to relieve at 130 psig; the toluene vessels
are relieving at 150 psig. Notice that the temperatures are approximately
the boiling points at the relieving pressures. Conventional relief valves
have an allowable back pressure of 10% of the set pressure.
3. Complete the Material Balance
Table 1 is completed for the common headers in the
manifold by carrying out a material balance. In Table 2,
the mass flow rates are added. A new entry is computed, molar flow rate.
Then, the temperature and viscosity of the combined streams are estimated by
using molar summations. This method of estimating the properties for the
mixtures is well within 5% of actual, and more than adequate for these
Refer to the Definitive Pipe Sizing formula. Both the upstream and downstream
pressures are within the right-hand-side. The equation cannot be rearranged to
solve for one pressure given the other. Therefore, the solution is iterative.
Modern spreadsheets have an equation solver feature that permits iterative
solutions to equations. In VentManifold, the Definitive Sizing formula is solved
in a Visual Basic subroutine to model the entire example manifold.
Relief valve discharge manifolds are needed to keep costs down and protect
the environment. Sizing them is straightforward but can be time consuming. It is
important to realize that high pressure drops in the piping require that the
iterative formula for compressible flow be used.
American Petroleum Institute, "Recommended Practice for
the Design and Installation of Pressure Relieving Systems in Refineries,"
4th edition, RP-520, API (1976).
Coker, A.K., "Size Relief Valves Sensibly," Chemical
Engineering Progress, 88:8, p.20 (August 1992).
Coker, A.K., "Determine Process-Pipe Sizes," Chemical
Engineering Progress, 87:3, p.33 (March 1991).
National Fire Protection Association, "Flammable and
Combustible Liquids Code," NFPA-30.
(1) Fisher, H.G., "An Overview of
Emergency Relief System Design Practice," Plant/Operations Progress,
10:1 (January 1991).
(2) Shaw, D.A., "SAFIRE Program for
Design of Emergency Pressure Relief Systems", Chemical Engineering
Progress, 87:7, p.14 (July 1991).
(3) Grossel, S.S., "An overview of
equipment for containment and disposal of emergency relief system
effluents," Journal of Loss Prevention in the Process Industry, 3:1,
p.112 (January 1990).
(4) Mak, H.Y., "New method speeds
pressure-relief manifold design," The Oil and Gas Journal, p.166
(Nov 20, 1978).
(5) Reid, R.C., Prausnitz, J.M., Poling,
B.E., The Properties of Gases and Liquids, 4th edition, McGraw-Hill
This is the basic data needed for chemengsoftware's
The material balance is completed by summing branches into subheaders, and
subheaders into the discharge header. VentManifold does these calculations