R. Merriam and G. W. FELS, Curtiss-Wright, Brecksville, Ohio
Extensive guidance is available for relief
protection of American Society of Mechanical Engineers (ASME) Boiler &
Pressure Vessel Code (B&PVC) Section VIII process equipment for scores of
applications, such as run-away reactions, distillation and heat exchangers. Sources
include ASME, American Petroleum Institute (API) and the Design Institute for
Emergency Relief Systems (DIERS), among others.
Vapor/steam relief protection from overpressure
for fired boilers is covered thoroughly in ASME Section I. However, there is
little-to-no guidance for sub-cooled or saturated/flashing liquid relief
protection for ASME Section I fired boilers and associated equipment.
This article will discuss applications that
require liquid or flashing liquid relief protection for ASME Section I
equipment, along with applicable code references and relief equipment used for
such protection. Applications discussed include economizers, low-quality steam generators
and liquid overfill of steam drums.
Relief equipment is certified for protection of ASME code equipment by the
National Board of Boiler and Pressure Vessel Inspectors (National Board). The
National Board certifies the device’s operation and capacity follow the performance
requirements defined by the construction code. All issued certifications are
listed within the published NB-18 “Redbook”1 listing the applicable
ASME Code Section to which the device is certified. An example of a partial
listing from NB-18 is shown in FIG. 1.
A device’s listing details the manufacturer,
model series, Code Section, NB capacity certification number, test media,
certified discharge coefficient, and ASME flow area certified according to the
coefficient of discharge method. Another certification method uses a slope or
flow factor to determine the relieving capacity.
Relief Equipment Certified for Section I Applications
Four main different types of relief equipment are
available with ASME Section I certification and are detailed in the following
ring safety valve (V-designator). The dual-blowdown ring safety valve marked
with V-designation is the workhorse for relief protection for ASME Section I
fired boilers. Most code boilers utilize these designs, and all but the
smallest boilers typically use two or more dual-ring valves certified for
Section I. These valves are characterized by full lift at low overpressure (3%
or 2 psi) and low blowdown (4%–6%). With the dual-blowdown rings and stiffer spring rates used
to achieve these strict requirements, they typically demonstrate unstable
operation when flowing liquid. These valves are only certified under vapor/steam
flowing conditions and are not expected to achieve full lift at 10%
overpressure, as required for liquid certification. These safety valves are
typically suitable for backpressures of up to 20% for the common open bonnet
FIG. 2 illustrates a cutaway view of a dual-blowdown ring safety
valve showing details of the upper and lower blowdown rings. The two rings are
required to simultaneously meet the 3% overpressure requirement and the 4%
blowdown requirement. These are suitable for steam/vapor operation but not
safety valve (V-designator). A V-designated safety valve with a single-blowdown
ring is available in accordance with ASME Section I. This valve is typically for
use with Section I economizer applications and organic vaporizer service
applications only. The capacity is certified for vapor/steam relief at 3%
overpressure; however, the blowdown requirement of 4% is not achieved. This
valve design is not intended for Section I boiler drum, superheater or reheater
safety relief valve (UV-designator). UV-designated safety valves with dual
certification in accordance with ASME Section VIII service have a single-blowdown
ring design, as shown in FIG.
3. These valves are characterized by full lift at, or below, 10%
overpressure for vapor or liquid applications and a blowdown of 7% in vapor
service. Valves may be certified for vapor flow, liquid flow or dual media.
While there is no direct certification for two-phase/flashing flow, a certified
capacity can be determined by guidance outlined by DIERS and API. The design of
the huddling chamber for the dual-certified valves results in stable operation
for vapor, liquid and two-phase flowing conditions. The National Board recently
added ASME Section I certification for liquid flow at 10% overpressure. For
conventional safety relief valves with a UV-designator, the amount of variable
superimposed and built-up backpressure is allowed up to the allowable overpressure.
For balanced bellows valves, the backpressure is limited by their backpressure capacity
curves, typically extending up to 50% or higher.
pilot-operated safety relief valve (V-designator). Invoking ASME Code
Case 2446 allows for a pilot-operated safety relief valve with a V-designation
to be certified for dual steam and liquid service. Vapor capacities are
certified at 3% overpressure and liquid capacities are certified at 10%
overpressure. While there is no direct certification for two-phase/flashing
flow, a certified capacity can be determined by guidance outlined by DIERS and
APPLICATIONS FOR ASME SECTION I SERVICE
for ASME Section I Boilers are heat exchangers that recover heat generated from
boilers and other fired equipment by preheating the boiler feedwater using the exiting
hot flue gases. From the Preamble of ASME Section I, “Superheaters, economizers, and other
pressure parts connected directly to the boiler without intervening valves
shall be considered as parts of the boiler proper, and their construction shall
conform to Section I rules.” When located outside the limits of a boiler’s
external piping, an economizer (FIG. 4) is considered a fired pressure vessel.
Economizers that can be partially or totally
isolated from the boiler require one or more pressure relief valves sized for the
discharge of saturated steam flow at a calculated capacity (lb/hr) using the
economizer’s stamped duty (Btu/hr) divided by 1,000 (Btu/lb). If overpressure
can occur within the economizer due to liquid, the pressure relief valve(s) are
also sized to limit the overpressure to no more than 10% or 3 psi, whichever is
totally isolated economizers may be designed in accordance with rules from
either ASME Section VIII, Div. 1 requirements or ASME Section I requirements (see
ASME Section I, Part PFE).2 Economizers constructed to ASME Section
VIII shall have overpressure protection in accordance with ASME Section XIII requirements
with a UV-designation.
As ASME Code does not provide
direct reference to overpressure protection for ASME Section I economizers that
can be partially or totally isolated, it is implied they are to follow ASME
Section I requirement.
The rules and requirements for economizers are
Low-quality steam generators. A second application involving ASME Section I
equipment requiring liquid/two-phase relief is a low-quality steam generator (FIG. 5). The
overpressure failure case in this situation would be a blocked outlet evaluated
at the design capacity and steam quality.
No ASME code reference has been identified for
this application. Possible relief protections to consider include:
Note: In situations where
the steam generator is designed to ASME Section VIII rules, protection may be
provided by a dual-certified safety relief valve with UV-designation.
Dual-certified pilot valves with V-designation
are specifically authorized for protecting Section I boiler economizers. The
manufacturer must be consulted to determine if they would be approved for
protecting low-quality steam generators.
Safety valves certified for ASME Section I
service are specifically designed for all vapor relief and may experience
stability issues if used to relieve two-phase mixtures like low-quality steam. Again,
the manufacturer must be consulted to determine if satisfactory
performance/stability can be expected.
Fired boiler steam drum. Liquid overfill of a fired boiler is one potentially
applicable relieving scenario, as illustrated in FIG. 6. ASME Section I PG-67 discusses
relief protection requirements for fired boilers and associated equipment. However,
the only reference identified for relief protection involving the boiler proper
identifies a blocked outlet at design steam capacity of the boiler.
ASME Section I boilers are typically protected
by dual-blowdown ring safety valve(s) with V-designation. As discussed earlier,
these safety valves do not provide stable operation when relieving liquid due
to the design of the huddling chamber and stiffer spring. These dual-ring
safety valves have no certified capacity for liquid flow.
As ASME only calls for steam relief associated
with the boiler proper and boiler safety valves are only certified for
steam/vapor flow, it is common for owner-operators to consider only the blocked
outlet steam rates. Dual-certified pilot valves with V-designation might be
used to relieve credible liquid scenarios for the boiler but are not certified
to protect the boiler proper for the steam blocked outlet scenario. Many safety
relief valves in the market are certified for liquid flow in ASME Section VIII
applications and are also certified for Section I liquid applications with a
V-designator. These can also be used to relieve the liquid overpressure
scenarios for ASME Section I boilers, but not for a steam blocked outlet case.
Takeaways. Four different designs
of relief protection equipment have been discussed, detailing characteristics
and their applicability for liquid relief protection for ASME Section I
The only liquid overpressure scenario called
out in code for ASME Section I equipment is associated with a boiler economizer
that can be partially or totally isolated from the boiler proper. Economizers
that can be isolated require overpressure protection for steam flow (lb/hr) at
their design duty (Btu/hr) divided by 1,000 (Btu/lb). Economizers designed
following ASME Section I or ASME Section VIII that can experience overpressure
by liquid, require overpressure protection for liquid relief.
Section I economizers are protected at 3%
accumulation for steam relief and 10% accumulation for liquid relief. For vapor
relief, a dual-ring safety valve, a single-ring safety valve, or a dual-certified
pilot valve are appropriate—all
with a V-designation. For liquid relief, a liquid certified safety relief valve
is required. This could be a liquid or dual-certified safety relief valve with
both V- and UV-designation or a dual-certified pilot-operated safety relief
valve with V-designation.
Section VIII economizers are protected at 10%
accumulation for steam relief and liquid relief. A dual-certified relief valve
with UV-designation could be used for this application.
ASME Section I does not address overpressure
protection for liquid relief associated with other equipment/applications, such
as low-quality steam generators or liquid overfill of steam drums.
Relief protection for a low-quality steam generator
is not specifically defined. Facility owner-operators should evaluate possible
options for overpressure protection, including dual ASME Section I certified
pilot relief valves, a V-designated safety valve for an ASME Section I steam
generator, or a UV-designated dual-certified safety relief valve if the steam
generator is designed to ASME Section VIII.
Liquid relief from boiler overfill is not
usually considered a creditable scenario. It might be possible to relieve the
liquid case with a dual ASME Section I certified pilot relief valve, but the
boiler steam blocked outlet scenario must be relieved by a dual-blowdown ring
safety valve with V-designation, as the pilot valve is not certified for steam
overpressure protection of the steam drum. A liquid- or dual-certified safety
relief valve with UV-designation also certified for liquid on Section I equipment
may also provide liquid relief protection for the boiler. HP
RUSSELL B. MERRIAM is a Senior Principal Engineer
for Farris Engineering Service and is actively involved with pressure relief
calculation studies, including contingency analysis, calculations for
distillation towers and other challenging calculations, audits of calculations,
and training on company proprietary softwarea. Merriam has been
using and assisting in program development for this softwarea since
he started working for Farris Engineering Services in 2004. He is an active
participant in the DIERS Group, and worked in the petrochemical industry for more
than 21 yr prior to joining Farris Engineering Services.
GEORGE W. FELS is a Senior Process Engineer
for Farris Engineering Services and is responsible for designing, sizing and
selecting emergency pressure relief systems, as well as software development. Fels
has also taught courses in overpressure protection management, and relief valve
design and applications. HeGeorge
holds BS and MS degrees in chemical engineering from the University of Dayton
and is an active member of the API Subcommittee on Pressure-Relieving Systems (SCPRS).