Guidelines for Engineering Design for Process Safety (Process Safety Guidelines and Concept)

630 Pages·2012·29.54 MB·English

14
PRESSURE RELIEF SYSTEMS
14.1 INTRODUCTION
This chapter concentrates on the design of emergency relief devices and
systems to minimize accidental releases of hazardous materials caused by
equipment overpressure or vacuum. High integrity equipment and pipingare
the first layer of containment. Depressuring (controlled release) is another
level of design to avoid large-scale releases. Pressure relief systems are the last
resort; therefore, they should be designed for high reliability even though they
will have to function infrequently. The design goal of all layers of containment
is to minimize the actuation of relief devices.
In recent years, many companies have incorporated the principles of
"depressuring" or ''instrumented shutdown" of key equipment as a means to
control a release and avoid the actuation of pressure relief devices. This
minimizes the probability of failure of the device, because once used the device
may no longer be dependable. Since maintenance of relief devices can be
sporadic, this redundancy (instrumented shutdown or depressuring) pro-
vides yet another layer of safety. Regardless of the number of levels of
containment and depressuring systems in place, overpressure protection must
be provided.
This chapter deals with causes of overpressure, relief devices available, and
problems encountered in sizing relief systems. Recent developments in the
analysis of two-phase flow venting are discussed in Section 14.6. This chapter
deals with relief systems where the effluent is handled by disposal to scrubbers
and/or flares, by containment, or by release to the atmosphere. Disposal of
effluents is covered in Chapter 15. Detonations and deflagrations are covered
in Chapters 13 and 17.
Regulations, codes, standards and guidelines which apply to the design of
pressure relief systems are listed in the References at the end of the chapter.
Many aspects of design are governed by regulation; specification of relieving
devices, relieving system design and discharge destination is often dictated
by governmental agencies to limit toxic or hazardous releases to the atmos-
phere.
Industry practice is to conform to the applicable regulations, codes, and
recommended practices. Often, these will provide different guidelines. A
prudent approach would be to review all applicable codes, standards, etc.,
prior to choosing a design basis. In addition, the Center for Chemical Process
Safety (CCPS), formed by the American Institute of Chemical Engineers
(AIChE), is developing guidelines and conducting research to further general
knowledge in emergency relief system design. The Design Institute for Emer-
gency Relief Systems (DIERS) was established by AIChE to address sizing
relief devices for two-phase, vapor-liquid flashing flows. In addition, most
engineering and manufacturing companies have developed internal engin-
eering standards which address specific safety concerns.
In the following sections pressure safety valves and relief devices are
addressed on the premise that the maximum allowable working pressure
(MAWP) and design temperature of the equipment being protected are proper.
Confirmation of this information would be done as a matter of course on a
new design and will be completed on revamps or retrofits in order to comply
with OSHA 29 CFR 1910.119 Process Safety Management of Highly Hazard-
ous Chemicals (where applicable). The relief device design must be consistent
with the system's temperature and pressure.
Storage tanks that operate at or near atmospheric pressure must receive a
critical review of potential causes of overpressure, equivalent to the review for
higher pressure vessels. NFPA 30 and API Standard 2000 provide guidance for
design of this type of overpressure protection. In particular, NFPA 30 focuses
on flammability issues, while API Standard 2000 focuses on both pressure and
vacuum vent requirements. A common tank failure scenario is insufficient
vent capacity (either pressure or vacuum) to allow for all operating cases plus
rapid climatic changes. Strict adherence to API Standard 2000 is strongly
recommended.
14.2 RELIEF DESIGN SCENARIOS
The designer of overpressure protection systems must consider all scenarios
that constitute a hazard under the prevailing conditions and evaluate them in
terms of the pressure generated and/or the rates at which the fluids must be
relieved. The scenarios under consideration may cause a release from a single
piece of equipment or from multiple equipment items. Overpressure may
result from a single failure or multiple failures, and the probability of occur-
rence of multiple events leading to relief should be considered in the design.
The scenarios leading to overpressure are discussed in this chapter under
several categories:
• fire
• blocked outlet
• operational failure
• equipment failure (hardware failure such as tube rupture or control
system failure)
• process upset, such as runaway reactions or excessive exothermic reac-
tions
• process causes, such as an imbalance of fluid flow rates
• utility failure
A detailed discussion of the causes of overpressure is given in API RP 521
and in the sections below.
14.2.1 Fire
The main result of fire exposure is heat input, causing thermal expansion or
vaporization or thermally induced decomposition, resulting in pressure rise.
An additional result of fire exposure is overheating a vessel wall to high
temperature in the vapor space where the wall is not cooled by liquid. In this
case, the vessel wall may fail due to high temperature even though the relief
devise is operating. Guidelines for calculating heat input are found in API
recommended practices, NFPA 30 (for bulk storage tanks), OSHA 1910.106,
and corporate engineering standards. In determining heat input from fire
exposure, NFPA allows credit for application of water to a tank; however, API
does not.
Pressure vessels (including heat exchangers and air coolers) in a plant
handling flammable fluids are subject to potential exposure to external fire. A
vessel or group of vessels which could be exposed to a pool fire must be
protected by a pressure relief device. Additional protection to reduce the
device relief load can be provided by insulation, water spray/deluge, or
remote-controlled depressuring device (control valve).
14.2.1.1 Determination of Fire Risk Area
Plant layout should consider spacing requirements such as those set forth by
NFPA, API, Industrial Risk Insurers (IRI) or Factory Mutual (FM) and must
include accessibility for firefighting (see Chapter 3, Plant Design). Several
pieces of equipment located adjacent to each other that cannot be isolated by
shutoff valves can be protected by a common relief valve, if interconnecting
piping is large enough to handle the relief load. The designer has to determine
which equipment items are affected in this fire risk area. With proper design,
API RP 521 considers a fire risk area as a plot which can be limited to between
230 and 460 m2 (2500 and 5000 ft2). Other design criteria may apply if a risk
area is located in a diked area which exceeds the API upper limits.
The surfaces of vessels or heat exchangers up to 9.1 m (30 feet) above grade
or other fire supporting level shall be considered subject to fire exposure, in
accordance with NFPA 30. It is usually assumed that all equipment is blocked
in and contains the operating liquid at normal maximum liquid levels.
The surface area exposed to fire determines the surface area effective in
generating or heating vapor; relief valve rate of discharge is affected by
whether or not the area is wetted by liquid (hence, boiling). See API RP 520,
Section 3 and Appendix D.
14.2.1.2 Determination of Relieving Capacities for Fire Contingency
Pressure relief capacity as described here is for thermally stable liquids such
as saturated hydrocarbons and not for reactive or thermally unstable systems
such as monomers. Methods for calculating the amount of vapor generated
are given in API RP 520.
Gas Expansion in Vessels Exposed to Fire
Vessels that are in gas service and are exposed to external fire require a
pressure relief device due to the thermal expansion of the gas. Depressuring
a gas-filled vessel should not be considered as an alternative to providing a
pressure safety valve for overpressure protection. The formulas for calculating
the required orifice area of the relief valves are given in API RP 520 for both
insulated and noninsulated vessels. It is important to note when taking credit
for insulation that the insulation must have the capability of withstanding
both fire and firewater impact (from fire hose). Upon exposure to fire, a
pressure vessel not protected with water sprays, fire water, or insulation may
overheat and fail within a few minutes.
Vapor Generation from Liquid-Containing Vessels Exposed to Fire
The amount of vapor generation from a vessel containing liquid and
exposed to external fire depends on the thermophysical properties of the fluid
inside the vessel, the relieving pressure and the heat input rate.
1. For a fluid that is below its critical temperature and pressure during
relieving conditions, vaporization due to an external fire will create a
volumetric expansion which may cause overpressure. The relieving rate
is equal to the vaporization rate. Note that if cold temperature insulation
is used on a vessel, the vessel is considered noninsulated (API RP 520),
unless the installation would satisfy fire protection requirements as dis-
cussed in Chapter 16. The total heat absorbed is a function of the vessel
dimensions, the liquid level and the insulation thickness, if any, of the
vessel. The heat absorbed from fire impact upon the wetted surface area
of the pressure vessel is calculated using the formula in API RP 520 or the
chart in NFPA 30. Note that for special conditions, such as where no
firefighting equipment or adequate drainage exists, specific equations
apply; the designer should refer to API RP 520 for more detailed calcula-
tion procedures for these contingencies.
API Standard 2000 and NFPA 30 provide equations for calculating
emergency relief venting for fire exposure for above ground tanks and
pressure vessels.
2. For a fluid above critical temperature and pressure during relieving
conditions, the relief valve orifice calculation becomes complicated. API
RP 520 provides guidance and formulas for calculating orifice size and
relief load calculations.
3. Two-phase flow can also occur in unique situations such as a bottom fire
on a vessel containing a fluid exhibiting foaming characteristics such as
latex (refer to the DIERS Project Manual for more information).
Fluids other than the normal process fluid (such as washing solvents) can
sometimes be found in a vessel, and such eventualities should be considered
when preparing the relief valve sizing calculations.
14.2.1.3 Allowable Pressure Accumulation for Fire Contingency
ASME Code Section VIII, Division 1, provides for allowable pressure rise for
fire contingency. Under appropriate conditions, a maximum relieving pres-
sure of 21% above maximum allowable working pressure (MAWP) is per-
mitted. Again, specifics should be confirmed after a thorough code review.
14.2.2 Operational Failure
The following scenarios of various operational failures may result in overpres-
sure conditions.
24.2.2.2 Blocked Outlet
Operation or maintenance errors (especially after a plant turnaround) can
block the outlet of a liquid or vapor stream from a process equipment item
resulting in an overpressure condition.
For the liquid blocked-outlet situation, the relieving load is typically the
normal flow unless the source is a mechanical equipment item such as a
compressor or pump. Examination of the characteristic curve of such equip-
ment may reduce the relieving load at the specific relieving conditions, that
is, set pressure plus overpressure. Alternatively, the system design pressure
(setpoint) maybe elevated above the maximum achievable operating pressure
if economics will allow.
For the vapor blocked-outlet situation, the relieving load is the maximum
vapor generation at the specific relieving conditions. This load maybe reduced
by taking credit for forward flow of vapor from the remaining vapor outlets
if they exist (e.g., a partial condenser in a fractionator overhead system).
14.2.2.2 Opening a Manual Valve
Manual valves which are normally closed to separate process equipment
and/or streams can be inadvertently opened, causing the release of a high
pressure stream or resulting in vacuum conditions. Additional side effects of
such a failure may include critical flows, flashing of liquid, runaway reactions,
etc., and must be considered as a consequence for this operating deviation.
14.2.2.3 flashing of Liquid Feed to Downstream Vessel
Control valves downstream of high pressure vessels containing liquid could
fail open resulting in excessive flow of liquid generating a high vapor flow to
the downstream vessel. Downstream vessels and equipment must be capable
of handling the excessive vapors; otherwise, relief is necessary.
14.2.2.4 Cooling Water Failure
One of the most commonly encountered causes of overpressurization is
cooling water failure. Two examples of the critical consequences of this event
are the loss of condensing duty in fractionator overhead systems and the loss
of cooling for compressor seals and lube oil systems. Different scenarios
should be considered for this event depending on whether the cooling water
failure affects a single equipment item (or process unit) or is plant-wide.
Examples of plant-wide scenarios include cooling water pump failure, failure
of any section of the main header or loss of fans on the cooling tower.
It is difficult to summarize the loads to be relieved from this failure. Most
companies have unique approaches to determining the relief rates from
towers, compressors, and the like. These approaches vary due to cooling
considerations, instrumented shutdowns, etc. API RP 521 discusses many of
these considerations.
14.2.2.5 Power Failure
Power failure will shut down all motor driven rotating equipment such as
pumps, compressors, air coolers, and reactor agitators. As with cooling water
failure, power failure can have a cascading negative affect on other equipment
and systems in the plant. Different scenarios should be considered for this
contingency depending upon whether the power interruption is local (to a
single equipment item), to a unit substation, or plant-wide. Adequate backup
features should be included in the plant's electrical design to reduce the
probability of a major power interruption to an acceptable level.
14.2.2.6 Instrument Air Failure
The consequences of instrument air failure should be evaluated in conjunction
with the failure mode of the control valve actuator. It should not be assumed
that the correct air failure response will occur on these control valves (fail
open, closed or in position). Some valves may stick in their last operating
position and the choice of which valves are assumed to fail in their last
operating position would be governed by maximizing the relieving load. If
the failure position of the valve helped minimize the relieving rate, it should
be assumed that the valve would not move. The relief valve size should be
based on the failure of the most critical component of the air supply system.
Following determination of the consequences of air failure, the designer
should size relief devices based upon specific knowledge of the system
including control valve flow characteristics and piping layout.
14.2.2.7 Reflux or Recirculation Failure
The loss of reflux or recirculation on fractionation towers is typically caused
by power failure to the pumps, a pump trip, or when a control valve fails
closed. The relieving rates should be analyzed based upon heat balances
around the fractionator to account for the loss of this heat sink. Generally, it
is assumed that overhead condensers are flooded and the gross overhead is a
conservative estimate for the relief rate. However, the analysis should con-
sider loss of fractionation effect on composition, reboiler temperature changes,
etc, as discussed in API RP 521. In addition, one might consider calculating
the effect of suppression of vaporization which occurs at the relief device
maximum relieving pressure. This suppression will typically give lower
estimates of the required flow.
14.2.2.8 Thermal Expansion
Equipment or pipelines which are full of liquid under no-flow conditions are
subject to hydraulic expansion due to increase in temperature and, therefore,
require overpressure protection. Sources of heat that cause this thermal ex-
pansion are solar radiation, heat tracing, heating coils, heat transfer from the
atmosphere or other equipment. Another cause of overpressure is a heat
exchanger blocked-in on the cold side while the flow continues on the hot side.
Cryogenic systems are particularly vulnerable to such failures.
14.2.2.9 Vacuum
Equipment may inadvertently experience vacuum caused by the following
contingencies:
• Instrument malfunction.
• Draining liquid from equipment without venting or gas repressuring.
• Shutting off purge steam without pressuring with noncondensable
vapors, for example, air, nitrogen, or fuel gas.
• Extreme cold ambient temperature resulting in subatmospheric vapor
pressure of certain materials.
• Loss of heat input to a process vessel handling low vapor pressure
material while simultaneously maintaining cooling, condensing or loss
of heat from vessel to ambient.
• Loss of heat to waste heat boilers with resulting steam condensation.
• Absorption process, for example, HCl vapors into water.
• Rapid climatic changes.
• Water addition to vessels that have been purged with steam.
Methods of protection against vacuum conditions caused by the above
contingencies may include:
• Design equipment for full or partial vacuum conditions.
• Install vacuum relief devices (avoiding explosive mixtures if air is used).
14.2.2.10 Absorbent Medium Failure
In certain processes it is required that entrained gases be removed from liquids
to avoid overpressure from accumulation of such gases and avoid upsets in
downstream equipment. For example, a lean oil absorption system is often
used for hydrocarbon services. In the production of CO-f ree hydrogen, carbon
dioxide is removed before the hydrogen rich gas enters the methanator. In
these cases loss of absorbent medium can cause overpressuring or excessive
methanation reaction, and an evaluation of the system is required to deter-
mine if relief protection is warranted.
14.2.2.11 Loss of Motive Steam to Ejectors
This contingency is specific for ejectors used in vacuum services (e.g., vacuum
towers in refineries). One scenario is the loss of motive steam which will, in
effect, over pressure the towers. Relief valves are always provided on such
towers and the relief load is generally considered as the sum of the process
steam, overhead cracked gases and noncondensables.
14.2.3 Equipment Failure
This section addresses common equipment failures that may result in over-
pressure or vacuum relief requirements.
14.2.3.1 Heat Exchangers: Tube Rupture
The ASME Code states that "heat exchangers and other vessels be protected
with a relieving device of sufficient capacity to avoid overpressure in case of
internal failure." Characterization of the types of failure, determination of
relieving capacity required, and selection and location of relief devices are left
to the discretion of the designer. API RP 520 presents guidance in determining
these requirements, including criteria for deciding when a full tube rupture is
a likely contingency.
The relieving rate for tube rupture is commonly based on the assumption
that one tube ruptures and provides two tube cross-sectional flow areas for
material to flow from the high pressure side to the low pressure side. This
material could be either vapor or liquid, with the phase determining the final
relieving rate. Careful attention to two-phase flow and flashing fluid con-
siderations is critical to the proper sizing of any relief device. For instance,
high-pressure gas or flashing liquid on the tube side and low-pressure liquid
on the low pressure side presents a very difficult relieving situation, which
may require a rupture disk device to obtain the needed quick relieving
response.
When calculating the flow out of the low-pressure side, credit can be taken
for the fluid handling capacity of both the inlet and outlet lines unless either
contains check valves or control valves which would tend to be closed by the
effects of tube breakage. See API RP 521. Since pressure shock could occur on
the low pressure side following tube failure, the proper placement and selec-
tion of the type of relief device [rupture disk] should be given due considera-
tion.
The "two-thirds" rule is frequently used to establish the low pressure side
design pressure as at least two-thirds of the high pressure side. The relief valve
on the low pressure side does not have to be sized/checked for the ruptured
tube case.
14.2.3.2 Heat Exchangers: Air Cooler Failures
There are two failures that commonly occur in air coolers, either fan failure or
louver failure. A louver failure (closure) may be the result of a control failure,
mechanical coupling breakage, or excessive vibration. This is considered a
total loss of cooling/condensing and therefore the relieving rates are calcu-
lated using total loss of coolant as described in Section 14.2.2.4, Cooling Water
Failure. This is a localized failure, however, and can sometimes be corrected
quickly enough to avoid loss of production which might introduce other
potential safety problems.
The loss of a fan is less detrimental due to continued natural convection
effects. API RP 521 recommends that, in condensing service, partial credit
between 20 and 30% of normal duty of the air cooler be taken. The relieving
rate is then calculated using the remaining 70 to 80% of the duty. When
practical, the designer should calculate the natural convective heat transfer
rate for each case.
14.2.3.3 Automatic Control Valve
The design premise of the facility should include requirements for overpres-
sure protection due to control valve failure. Two scenarios could be evaluated
for this contingency:
• Failure of control valve in wide open position causing a high pressure
fluid to enter a lower pressure system. This may result in partial flashing
of fluid across the control valve causing two-phase flow.
• Failure of control valve in closed position (blocked inlet or outlet).
If a bypass valve has a larger valve coefficient, C than the automatic control
v/
valve, consider flow through the bypass for relief load calculation.
14.2.4 Process Upset
14.2.4.1 Runaway Reaction
Runaway temperature and pressure in reactor vessels can occur as a result of
several factors. Some of these are loss of cooling, feed or quench failure,
excessive feed rates or temperatures, runaway polymerization, contaminants,
catalyst problems, or instrument and control failures (e.g., agitation failure).
The main concern here is the high rate of energy release and/or formation of
gaseous products which may cause a rapid pressure rise in the reactor. The
consequence of high vessel temperatures is a reduction of the allowable stress
in the vessel. There are no general rules for determining the relief loads for
this contingency. Design of adequate emergency relief requires a knowledge
of heats of reaction, products of reaction, pressure-temperature relationships,
and kinetics for both normal and upset conditions.
Before designing relief for overpressure, modifications in the process
should be considered to see if the inherent safety can be improved (Chapter
2, Designing Inherently Safer Processes). These modifications might include:
• reduce amount, concentration or fill fraction of reactants, initiators or
contaminants
• change operating temperature or pressure
• increase amount of solvent
• increase or modify emergency relief system
• redesign the process
After the inherent safety of the reaction is maximized, various protective
methods can be incorporated into the design of a system such as:
• A higher margin in the design temperature or pressure of the equipment.
• Monitors and controls to mitigate runaway temperatures.
• High temperature shut downs or feed trip.
• Rapid vapor depressuring by remote controls.
• Addition of volatile fluids to absorb excess heat of reaction.
• Recycle of reacted product to dilute the feed.
• Addition of inhibitors to monomer systems.
• Addition of catalyst poison to kill the reaction.