Evaluate Pressure Relief System

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Evaluate pressure relief system forces in existing installations

01.01.2013 | Smith, D., Smith & Burgess LLC, Houston, Texas; White, J., Smith & Burgess LLC, Houston,

Texas; Frenk, B. , Frenk Water Technologies LLC, Byron, Illinois

The purpose of this study is to limit the number of relief valves that require rigorous engineering calculations to

determine the adequacy of the installation.

Keywords:

Pressure relief devices control the amount and disposition of material during a process upset while

simultaneously protecting process equipment from damage due to overpressure caused by the upset. The

devices most commonly used for these purposes are pressure relief valves (PRVs) and pressure safety valves

(PSVs).

Quite a bit of engineering research, testing and analysis has been performed to improve assessment of relief

valve suitability and the ability of associated installations to protect equipment from overpressure. One area

that has less prescriptive requirements is analysis of the structural integrity of the relief device installation

during the emergency event.

These installations are not designed for continuous flow, but rather sporadic flow, often at choked or sonic

conditions. This article takes a brief look at the existing evaluation of reaction forces for PRVs. It also performs a

detailed baseline analysis of typical installations to develop a screening tool for evaluating an existing facility,

and identifies the results when tested against an existing petrochemical facility.

The purpose of this study is to limit the number of relief valves that require rigorous engineering calculations to

determine the adequacy of the installation. Often, it is assumed that PRV installations are simple and easy to

design. However, practical experience has shown that PRVs, particularly devices that discharge to the

atmosphere, are the most easily manipulated during the actual construction phase and often are not installed as

they were intended.

See Fig. 1 for two examples of installations that were likely not installed as designed. Clearly, not all existing

relief valve installations meet the specifications recommended by industry practice.

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Fig. 1. Examples of PRV installations where

little or no engineering piping design was

performed or design was not followed.

Reaction force analysis methodology

During an overpressure event, the discharge of a PRV imposes a load, referred to as a reaction force, on the

collective installation. This force creates a bending moment that is both a function of the quantity and state of

the release and the physical layout of the piping installation (i.e., the lever arm created by the installation). The

stress caused by the reaction force is propagated into and through the PRV and then into the inlet piping and

vessel nozzle, unless the system is properly supported.

API reaction force analysis. The American Petroleum Institute (API) provides guidance for determining

pressure relief requirements for pressure relief device installations. API 520, Part 2, states that PRV outlet

piping should be independently supported and properly aligned. Stresses due to forced directional alignment of

PRV piping are also mentioned; however, that topic will not be discussed in this article. The authors collective

practical experience has demonstrated that a significant portion of atmospheric relief devices do not have piping

supports in place, as described in Fig. 2.

Fig. 2. Recreation of figure from API 520 for

a typical relief valve installation. Note: The

support should be as close as possible to

the centerline of the vent pipe. F = the

reaction force, and A = the cross-sectional

area of discharge pipe.



API 520, Part 2, provides a basis calculation for the reaction forces in the event of vapor or two-phase releases

directly to the atmosphere. There is no discussion in this section of the reaction forces developed during a liquid

release. Furthermore, no guidance is presented with respect to applying these results or determining if an

installation is acceptable; instead, the burden is placed on the designer to ensure that the installation is

appropriate. While this may be reasonable for the design of new facilities, evaluating the adequacy of existing

facilities becomes much more complicated.

The formulas from API 520, Part 2, are listed below for relief devices discharging to the atmosphere:

API 520, Part 2, 4.4.1.1: Customary units for vapor relief reaction forces:

(1)

API 520, Part 2, 4.4.1.2: US customary units for two-phase relief reaction forces:

(2)

where:

F = Reaction force at the point of discharge to the atmosphere, pound force (lbf)

k = Ratio of specific heats (CP/CV) at the outlet conditions

W = Flow of any gas or vapor, pound mass (lbm)/hr

CP = Specific heat at constant pressure

CV = Specific heat at constant volume

T = Temperature at the outlet, R

M = Molecular weight of the process fluid

A = Area of the outlet at the point of discharge, in.2

P = Static pressure within the outlet at the point of discharge, psig

x = Weight fraction vapor at exit conditions

g = Vapor density at exit conditions, lbm/ft3

l = Liquid density at exit conditions, lbm/ft3

Pe = Absolute pressure at pipe exit, psia

Pa = Absolute ambient pressure, psig.

DIERS reaction force analysis. The Design Institute for Emergency Relief Systems (DIERS) provides

similar guidance for the consideration of reaction forces and the determination of the acceptability of a relief

device installation. Additional recommendations are provided for a suggested piping layout to avoid excessive

lever arms, as recreated in Fig. 3. In this illustration, the system on the left has significantly more stress due to

the increased lever arm and direction of discharge when compared to the system on the right.



Fig. 3. Even the smallest modification in piping

design can have a significant effect on the

resulting reaction forces.

Additionally, emphasis is placed on the importance of evaluating the reaction forces for all credible

overpressure contingencies, not simply the controlling contingency. This is important because the physical

properties are not always the same, and, in some cases, the controlling contingency for sizing may be a vapor

stream, while the controlling case for the reaction forces may be a two-phase stream.

Reaction force case study analysis

A piping system may respond far differently to a dynamic load than to a static load of the same magnitude.

Static loads are those applied slowly enough so that the piping system has time to react and internally distribute

the loads, thereby remaining in equilibrium.

With dynamic loadsthose that change quicklythe piping system may not have time to internally distribute

the loads, so forces and movements are not always resolved, resulting in unbalanced and potentially

concentrated loads and pipe movement.

The typical action of relief valve venting is an impulse load, where the flowrate and associated forces ramp up

from nominally zero to some value, remain relatively constant for the duration of the release, and then ramp

down to zero again. When the relief valve opens, the discharge fluid creates a jet force that acts on the piping

system. This force increases from zero to its full value over a time frame similar to the opening time of the valve.

The relief valve remains open until sufficient fluid is vented to relieve the overpressure situation. When the

valve closes, the reduction in flow corresponds to the loss of the jet force over the closing time of the valve.

Multiple relief valve piping configurations were examined for both static and dynamic conditions, using analysis

software. The stresses calculated during the analysis were checked against the allowable stresses per American

Society of Mechanical Engineers (ASME) standards. Additionally, the analysis was used to determine if a flange

leak was likely. In all cases, the dynamic condition was determined to be the governing condition for the

structural integrity of the piping system.

The leakage check examined the tendency of the flanges to separate under the applied piping loads. ASME B31.3

does not directly address flange leakage. The purpose of this analysis was to determine piping failure and not

flange leaks.



From extensive tests, It has been determined that, even under unusually severe bending stresses, flange

assemblies did not fail in the flange proper, by fracture of the bolts or by leakage across the joint face. Structural

failure occurred almost invariably in the pipe adjacent to the flange, and, in rare instances, across an unusually

weak attachment weld. Leakage well in advance of failure was observed only in the case of threaded flanges.1

These findings, which suggest that the structural integrity of the piping is the major area of concern for any

stress analysis and should be considered over flange leakage, serve as the basis for these evaluations.

Many different process connections have been observed in field installations: Welding-reducing tees,

weld/thread o-lets, and unreinforced stub-in connections. Unreinforced stub-in connections result in the

highest ratio of calculated to code-allowed stresses, followed by o-lets and then welding-reducing tees for the

same relief system-applied piping loads. The model used in the current evaluations has been confined to

welding-reducing tees.

The allowable code stresses are below the yield and well below the tensile, as indicated in Table 1, for

commonly used carbon steel materials. The net result is that there is a 19% to 24% safety factor between the

code allowable for occasional loading and the yield point where the material begins to fail. The relief valve

models were evaluated to establish relief pressures at which the calculated stresses were within 5% of each of

the allowable occasional, yield and tensile stresses.

Modeling details. The relief valves were modeled as an open discharge, with a vertical pipe discharging

directly to the atmosphere. As shown in Fig. 4, the process connection is mounted on a pipe header with a

welding-reducing tee. This arrangement was chosen to provide a more realistic representation of typical

installations together with the inherent flexibility of the tee/header connection. The vent pipe is the same

diameter as the outlet connection on the valve and is unsupported at the elbow, with a 6-foot-long vertical vent

pipe.

Fig. 4. Sample of the model basis.



Another software program was used to determine the physical properties along the vent pipe required to

calculate the thrust and momentum forces (Fig. 5):

Average velocity along the vent pipe

Average temperature across the outlet of the vent pipe

Average velocity at the elbow.

Fig. 5. Sample of the velocity profile output.

The relief valve was modeled as an orifice at the end of a converging nozzle. The orifice was set to produce the

capacity calculated by a relief valve analysis for the given inlet conditions using the certified orifice size. The

following assumptions were made regarding the analysis:

The process fluid was vapor

The manufacturers certified orifice diameters (from the National Board of Boiler and Pressure Vessel

Inspectors Relief Device Certification NB-18) were used in place of standard API orifice diameters to

provide more realistic discharge flow

Valve opening and closing time was 8.0 milliseconds, and venting would last for one second; these

numbers are specific to the valve manufacturer, and they appear to be typical throughout the relief valve

industry

Wind loadings were not considered

All piping was considered to be schedule 40 carbon steel

Relief valve inlet flanges were specified as required for process considerations

Relief valve outlet flanges were specified to American National Standards Institute (ANSI) RF 150.

Screening study and results

The objective of developing the screening tool was to provide a fairly quick method to identify relief valves that

were likely to need either support or more detailed analysis to verify the adequacy of the existing installation.

For the purpose of simplification, several assumptions were made, as described below:

All relief valves discharging to a closed disposal system are adequately supported for an individual

release

Relief valve installations that discharge to a closed system are, by definition, supported by at least

the point of discharge, and the purpose of this screening is to identify relief valves that require

supportnot to evaluate the adequacy of existing supports

Due to the complexity of a supported common disposal system, these systems are excluded from

the scope of this study

All liquid and two-phase relief contingencies require detailed analysis



Water hammer is a much bigger concern for liquid and two-phase releases than for reaction

forces, and is therefore an item to be evaluated outside of reaction forces

Additionally, the dynamic effects of flashing flow create far too many variables to include in a

simplistic screening

All nonstandard PRV sizes require detailed analysis

While this statement may not be true for every installation, for the purposes of developing an

automated tool to identify relief valves that may need detailed engineering, the obvious decision is

to flag any installation that falls out of the normal range

For the purposes of this study, the standard was defined by the flanged relief valve sizes listed in

API 526 and shown in Fig. 6.

Fig. 6. Relief valve size configurations

evaluated as per API 526.

PRV installations can be characterized as either typical or complex. Since the generic screening methodology

was performed using a typical PRV configuration (as seen in Fig. 4), some method of identifying configurations

with more complex piping had to be identified. For the purposes of this study, any piping configuration

containing more than change of direction fitting (elbow, 45 bend, branch tee, etc.) was considered to be

complex. Fig. 3 was used as the basis for this assumption.

PRVs installed and sized for only the external fire contingency will not require a reaction force evaluation. While

this may seem counterintuitive, as external fire is the prevailing overpressure contingency in pressure relief

system design, it is proposed that a PRV installation cannot be deemed adequate by a reaction force analysis in

the event of an external fire.

The heating effect on the relief device from a fire is unknown, and it can be more significant for the relief device

installation (particularly the outlet piping and the valve body itself) than the stresses caused by the flowrate. It is

proposed that the failure of a relief valve installation due to reaction forces is more likely to be controlled by the

reduction in tensile strength of the installation due to the heat input, than to be controlled by the system design.

PRVs installed and sized for only the liquid hydraulic expansion contingency will not require a reaction force

evaluation. Liquid hydraulic expansion cases are often nominal rates for which a small thermal PRV is installed.

In many cases, the PRV possesses a capacity far greater than the required relief load. Additionally, the non-

steady-state nature of a thermal expansion event tends to result in the unsustained releases that do not develop

typical fluid flow characteristics. Taking these factors into account, it was determined that hydraulic expansion

scenarios do not require pipe stress screening.

Qualitative screening. The screening study was divided into two phases, one being qualitative screening

against assumptions, as set forth above, and the other being against the criteria set forth in the base-case study

for each relief valve size. The qualitative step was performed stepwise, as a discussion tree, as represented in

Table 2.



The qualitative screening step identified 32 installations that are acceptable as is, 45 that require more

detailed analysis (including the potential review of why two-phase/liquid releases are being sent to the

atmosphere), and 112 installations that are not covered by this screening.

The second stage of the screening was developed based on generic pressure relief device installations. The

intention was to draw a line separating PRV installations into three categories:

Installations predicted to be acceptable as installed

Installations that will require detailed engineering analysis to determine the adequacy of the installation

Installations that are expected to require proper piping support if a detailed engineering analysis is

performed; the results of this screening are shown in Table 3.

This screening was performed against all 189 relief device installations rather than the remaining 112 from the

qualitative screening, due to the fact that three possible results exist rather than the two previously used. A relief

valve installation that was flagged as needing a detailed analysis in the qualitative step may be identified as a

device that is predicted to require support irrespective of a detailed analysis. Additionally, a relief device sized

only for fire may be identified as an installation requiring support, even before the effects of temperature change

can be examined.

To perform this screening, the existing PRV installations were divided into typical and complex groups, as

described previously. For typical installations, a threshold value of 90% was used when comparing the

installation to screening tool-generated stresses for each relief valve size. This means that relief valves having a

relief pressure within 90% of the threshold for occasional loading were flagged as requiring detailed engineering

analysis. Similarly, relief pressures exceeding 90% of the threshold for yield stress were flagged as likely

requiring support, regardless of the detailed analysis. For complex PRV installations, the threshold value was

lowered to 70% of the occasional loading and yield stress limits, respectively.

The results of the quantitative screening, shown in Table 3, indicate that most of the valves predicted to

require support (28 of the 37 identified) fall into the category of those predicted to exceed the yield stress.

Therefore, these valves were identified as installations that do not require detailed analysis to determine if

support is required.

Overall results. The results of both screening studies were combined to create an actionable list of items for

ensuring the physical integrity of the PRV installations, as shown in Table 4. For the facility studied, the

aggregate of the two screenings predicted that 28 pressure relief installations would require support (even if a

detailed engineering study was performed), and an additional 34 PRV installations would require a more

detailed engineering study to determine the adequacy of the installations. Overall, more than 30% of the relief

valves studied were found to require action.



A sample was taken from each of the three categories, and detailed analyses were performed to verify these

results. Of those samples, all relief device installations predicted to require support did indeed require support

to avoid exceeding the yield stress. Likewise, all sampled installations predicted to be adequate were found to be

adequate. Of the sampled devices predicted to require detailed engineering analysis, all but one exceeded the

yield stress, and that installation did exceed the allowable stress.

The purpose of this study was to provide a solid screening tool to prevent the cost of performing a detailed

engineering evaluation on every relief device installation, and the end result met this objective. In some cases, it

may be more cost-effective to simply support PRV installations for which a detailed study is suggested, rather

than to perform the detailed study.

Recommendations

Overpressure protection analysis has evolved significantly since the inception of the process safety management

(PSM) standard, but the mechanical stress applied to the piping during overpressure events appears to have

been, for the most part, overlooked. Criteria for identifying pressure relief device installations that may exceed

allowable stress levels were developed from these systems. These criteria were then evaluated against a

petrochemical facilitys pressure relief systems and benchmarked for validity as a first-pass tool to identify

installations potentially requiring physical supports. For the facility studied, approximately two-thirds of the

PRV installations were predicted to be adequate with respect to reaction forces, with the remaining installations

being broken into two categories: those requiring support and those requiring further analysis.

This proves that, in practice, a significant percentage of PRV installations do not meet the desired structural

integrity with regard to reaction forces. This study demonstrates a screening tool that allows plants to focus

resources on the relief valve installations most likely to fail due to reaction forces. HP

LITERATURE CITED

1 Peng, L.-C. and T.-L. Peng, Pipe Stress Engineering, ASME, New York, 2009.

The authors

Jason White, PE, is a senior process engineer with Smith & Burgess LLC, a process safety

consulting firm based in Houston, Texas. Mr. White has seven years of experience in PSM

compliance, specializing in relief systems design and analysis for the refining, natural gas and



petrochemical industries. He received BS and MS degrees in chemical engineering from the

University of Missouri, and is a licensed professional engineer in the state of Texas.

Dustin Smith, PE, is the co-founder and principal consultant of Smith & Burgess LLC, a process

safety consulting firm based in Houston, Texas. As a consultant, Mr. Smith has extensive

experience with helping refineries and petrochemical facilities maintain compliance with the PSM

standard. He has more than a decade of experience in relief systems design and PSM compliance.

His experience includes both domestic and international projects. Mr. Smith is a chemical

engineering graduate of Texas A&M University and a licensed professional engineer in Texas.

Bill Frenk is a mechanical engineer and the owner of Frenk Water Technologies LLC, a

mechanical engineering consulting firm based in Byron, Illinois. He is also a consultant for Smith

& Burgess LLC. Mr. Frenk has extensive experience utilizing Caesar II software in both onshore

and offshore applications. He received a BS degree in mechanical engineering from Southern

Illinois University.

Joe Bucci

02.19.2013

I work in the HVAC industrie, how often do boiler PRV's need to be recertified? And do refrigerant PRV's need

to be recertified?

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