1 Scope ……………………………………………………………………………………………………………………2 General ………………………………………………………………………………………………………………….3 Overpressure Protection ………………………………………………………………………………………….TABLE I – Pressure Specifications for Commonly Used Fisher Rosemount Company Pressurereducing
Regulators
………………………………………………………………………………………………………..FIGURE
1
–
Typical
Piping
Diagrams
………………………………………………………………………………..Figure
1A.
Supply
Pressure
60
psig
or
Less
But
Greater
than
the
Maximum
Allowable
WorkingPressure
of
the
Low-pressure
System
……………………………………………………………………………….Figure
1B.
Supply
Pressure
Greater
Than
60
psig
………………………………………………………………4
Revision History ……………………………………………………………………………………………………..
1 Scope
1.1 This standard provides guidance in the design of pressure reducing facilities to
supply gas pressures in the range of 0 to 10 psig for applications such as natural gas
to burners and inert gas for blanketing storage tank vapor spaces.
1.2 This standard does not circumvent specific state or local code requirements where
they apply.
2 General
2.1 A shutoff valve shall be installed immediately upstream of each pressure regulator.
Where more than one regulator is used in series, a single shutoff valve upstream of
the combination will be adequate.
2.2
In general, a completely self-contained regulator with no external impulse lines
should be selected for these services. It shall be capable of reducing the distribution
system pressure to the level required by the user, and regulating it within the
necessary limits of accuracy. Where external impulse lines must be used, they shall
be provided with suitable mechanical protection and shall be adequately sized to
ensure rapid response.
2.3 The regulator throttle valve should be single port with an orifice diameter not
exceeding that recommended by the manufacturer for the maximum inlet pressure.
2.4 Regulators with resilient valve seats should be used. These shall be designed to
resist cutting by the valve and permanent deformation from being pressed against the
valve port and shall withstand the abrasion of the gas and any impurities it may
contain.
2.5 Self-contained regulators using a spring and diaphragm mechanism normally are
selected for these applications. These regulators are distinguished by their large
diameter, flat diaphragm cases.
2.5.1 A diaphragm case vent is necessary for proper operation of this type
regulator. Care should be taken to ensure this vent is not restricted or
plugged in any way during installation or operation. Where the regulator is
mounted in a building or an enclosure, this case vent should be piped outside
to a safe, remote location to avoid a possible unsafe condition should the
diaphragm develop a leak.
2.5.2 Bodies for these regulators shall conform to the design pressure,
temperature, and material of construction specified for valves in the
applicable piping code.
2.5.3 Although the body for this type regulator may be adequately designed for the
inlet pressure, the diaphragm case generally does not meet this pressure
rating. See Table 1. It must be assumed that any valve will leak through the
seat. Therefore, any blocked valves or blockage downstream of the regulator
may lead to full supply pressure on the low pressure side of the regulator
unless protected by a pressure relief device.
2.5.4 Where the maximum case pressure rating is not known, it must be obtained
from the regulator manufacturer.
2.5.5 A straight run of piping the same size as the regulator outlet should be run for
a minimum distance of 18 inches downstream of the regulator to ensure
proper performance.
2.5.6 The downstream piping must not be restricted. Piping that is too small or
temporary hoses used in maintenance shops can cause the regulator to
exceed the diaphragm case design pressure and rupture the case.
2.6 Where operating continuity during regulator servicing is required, a suitably piped
bypass shall be provided. See Figure 1.
2.6.1 A restricting orifice shall be provided in the bypass piping upstream of the
hand valve used to throttle the bypass flow. This orifice shall be sized to
restrict flow through the bypass to a value less than the capacity of the lowpressure
system
overpressure
protection
device.
2.6.2
A multiturn, globe body valve, meeting the applicable piping specification
requirements, shall be provided for throttling the bypass flow.
2.6.3 If the primary regulator diaphragm case pressure rating exceeds the supply
pressure, the downstream block valve may be located between the regulator
and the low-pressure system overpressure protection device. When the
supply pressure exceeds the diaphragm case pressure rating, the block
valve must be located downstream of the overpressure protection device and
additional overpressure protection must be provided for operating through
the bypass. See Figure 1.
2.7 A block valve shall be provided in the low-pressure line to the gas user where it is
necessary to permit firing on alternate fuel, or to bottle up a tank vapor space during
regulator servicing periods. Location of this valve relative to the low-pressure system
overpressure protection device shall be as recommended in 3.6.3. See Figure 1.
3 Overpressure Protection
3.1 Overpressure protection shall be provided to prevent the maximum pressure in the
low-pressure system from exceeding a value, which would cause unsafe operation or
failure of any connected and properly adjusted low-pressure gas burning equipment
or other connected device.
3.2 No overpressure protection is required downstream of the primary regulator when the
supply pressure does not exceed the maximum allowable working pressure of the
low-pressure system.
3.3
If the process vapor or gas is not flammable, toxic, or corrosive and the supply
pressure do not exceed the regulator diaphragm case pressure rating, a single
regulator with appropriate overpressure protection for the low-pressure system may
be used.
3.4 Where flammable, toxic, or corrosive vapors or gases are to be controlled,
overpressure protection shall be provided as follows.
3.4.1 When the supply pressure is greater than the maximum allowable working
pressure of the low-pressure system but does not exceed 60 psig, suitable
overpressure protection shall be provided downstream of the primary
regulator. See Figure 1A.
3.4.2 A first-cut pressure regulator, with its own overpressure protection device,
shall be used in addition to a primary pressure regulator and suitable
overpressure protective device when the supply pressure exceeds 60 psig.
The first-cut regulator shall be set to maintain a pressure not exceeding 60
psig. See Figure 1B.
3.5 Overpressure protection may be accomplished by any one of the following means:
a. A relief valve of a type meeting the provisions of the ASME Boiler and
Pressure Vessel Code, Section VIII, Division 1. This means is easiest to fit
into existing protective device maintenance programs.
b. A first-cut regulator with its own overpressure protective device installed
upstream of the primary regulator and set to limit continuously the pressure
at the inlet of the primary regulator to the maximum allowable working
pressure of the downstream piping system. Recommended when the supply
pressure exceeds 60 psig.
c. An automatic shutoff device installed in series with or included as an integral
part of the regulator design, set to shut off when the pressure on the
downstream piping system reaches the maximum allowable working
pressure or some other predetermined pressure less than the maximum
allowable working pressure. For natural gas burner systems, this type of
device must be designed to remain closed when tripped until manually reset.
d. A liquid seal relief device, such as a dip tube submerged in a liquid, which
can be set to open accurately and reliably at the desired pressure. It is
limited to systems with very low set pressures.
3.6 The overpressure protection device may be installed as an integral part of a regulator
or as a separate unit. In either case, it shall be sized to handle full regulator capacity
and shall be installed in such a way as to accommodate testing on a regular basis.
3.7
Discharge stacks, vents, or outlet ports of all pressure relief devices shall be located
where gas can be discharged into the atmosphere without presenting a hazard.
3.8
Discharge piping shall be provided with an open drain to prevent accumulation of
rainwater or liquid. Where required, provision should be made to prevent birds and
insects from building nests in the discharge piping.
TABLE I – Pressure Specifications for Commonly Used Fisher
Rosemount Company Pressure-reducing Regulators
FIGURE 1 – Typical Piping Diagrams
Figure 1A. Supply Pressure 60 psig or Less But Greater than the
Maximum Allowable Working Pressure of the Low-pressure System
Figure 1B. Supply Pressure Greater Than 60 psig
