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SAFELY SIZE AND DESIGN

Relief Headers

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.

PIPE SIZING

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 2).

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 prediction models.

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.

ECONOMIC CONSIDERATIONS

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.

SYSTEM DESIGN

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 ports regularly.

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 tests.

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 < 100F) 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 "knock-out" pots.

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 separate page.

Initial Calculations

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. See an example.

2. Basic Data

Make a table with basic data. See example 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 segment.

Actual or proposed pipe diameters are entered. The example uses nominal pipe dimensions, but actual inside diameters will make the calculations more accurate.

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 example, 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 calculations.

Definitive Calculations

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. 

SUMMARY

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.

REFERENCES

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 (1987).

Table 1

This is the basic data needed for chemengsoftware's VentManifold spreadsheet template.

Table 2

The material balance is completed by summing branches into subheaders, and subheaders into the discharge header. VentManifold does these calculations automatically.

 

 

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