Sizing of Independent Pressure Regulators for Large Propane Systems

Transcription

1 SUPPLEMENT 3 Sizing of Independent Pressure Regulators for Large Propane Systems Editor s Note: Supplement 3 provides guidance to designers and installers of larger propane systems over 5, Btu/hr (147 kw/hr) that use a pressure relief device separate from the regulator. The editor thanks Mark Hood, who wrote this supplement while an employee of Fisher Controls International, LLC. Mr. Hood is an engineer who designs regulators and other propane valves and equipment. He has written a supplement that provides practical guidance that did not previously exist in this usable form. The editor also thanks Fisher Controls for permission to use the table and exhibits included in this supplement. The selection and installation of pressure regulators for propane piping systems is specified in and of NFPA 58. Pressure relief valves are required to be integral with the regulators and operated in accordance with the pressure limits established in UL 144, Standard for LP- Gas Regulators [1]. However, regulators typically found in industrial or commercial applications may not have sufficient integral relief protection to comply with the 2 psi overpressure protection equipment. Section 6.8.2(E) permits the use of an independent pressure relief valve when a regulator larger than 5, Btu/hr (147 kw/hr) without an integral pressure relief valve is installed. The independent pressure relief valve must limit the outlet pressure of the second-stage regulator to 2 psi (14 kpa). To meet this performance requirement, an engineering calculation must be performed, and performance curves of the equipment used are needed. In a typical system described in 6.8.2, standard residential-sized regulators easily accommodate the fuel demand. In most manufacturers product offerings, first-stage regulators incorporate a relief valve as required by and have outlet pressures limited to 1 psi (69 kpa) or less in conformance to Similarly, the second-stage regulators incorporate an integral relief valve meeting the overpressure performance requirements of UL 144. Therefore, when installed together to form a two-stage system, the inlet and outlet operational pressure ranges of both regulators have been satisfied, and the system overpressure performance is assured. In contrast, these parameters cannot be assumed when choosing components for engineered systems, but must be carefully selected based on inlet and outlet pressure ratings, orifice sizes, and capacities of integral or external relief valves. The selection sequence for a basic industrial two-stage installation utilizing supplemental relief valves can be described by the following: 1. Determine the interstage pressure. The engineered system described by 6.8.2(E) is not limited to the 1 psi (69 kpa) maximum interstage pressure required by for typical residential installations. The components of the system must be chosen based on the length of pipe required between the locations of the storage tank(s) and the load; the preferred pipe size and, therefore, its capacity and pressure drop; consideration of low-temperature recondensation into liquid; and any applicable local codes. 2. Select the first-stage regulator to provide the desired flow capacity and interstage outlet pressure. Attention should also be paid to ensure that its inlet pressure rating is at least equal to maximum propane tank pressure of 25 psi (1.7 MPa) and that its flow capacity is adequate at the lowest anticipated wintertime inlet pressure from the tank or vaporizer. 3. Select the second-stage regulator to provide the de- 527

2 528 Supplement 3 Sizing of Independent Pressure Regulators for Large Propane Systems sired flow capacity and needed outlet pressure at the given inlet pressure supplied from the outlet of the first stage less any line pressure drop. 4. Select the relief valve for the second stage with a start-to-discharge set pressure and exhaust capacity to ensure that the pressure to the downstream appliance controls never exceeds 2 psi (14 kpa). Both 6.8.2(F) and (3) specify that the second-stage regulator limit downstream overpressure to 2 psi (14 kpa) or less in the disc removed failure mode. This simulated failure mode was selected as a way to standardize testing for residential-sized regulators with integral relief valves. Paragraph 6.7.3(F) also serves as the minimum benchmark for industrial regulators. However, industrial-sized regulators often do not incorporate integral relief valves, and, therefore, their relief valve sizing information is usually reported in manufacturer s catalogs in terms of wide-open orifice coefficients. This worst-case assumption allows the selection of a separate downstream relief valve with an exhaust capacity greater than the flow that can pass through the regulator s orifice in a wide-open condition at the maximum anticipated inlet pressure. 5. Select the relief valve for the first stage to ensure that the interstage pressure is never higher than the maximum allowable operating inlet pressure of the secondstage regulator. Again, this selection is made by ensuring that the relief valve exhaust capacity is greater than the flow that can pass through the wide-open orifice of the first-stage regulator at the maximum inlet pressure of 25 psi (1.7 MPa). This selection can be determined from manufacturers catalogs. Due to the large flow capacities of industrial-size regulators in engineered systems, consideration should be given to the potential hazard posed by the use of equally large relief valves in the planned installation location. If relief valves are not acceptable, other methods of providing overpressure protection, such as monitoring regulator systems or automatic shutoff devices, are allowed. (See the commentary following ) EXAMPLE An example of sizing an independent pressure relief valve is provided here to assist in understanding the concept. In order to provide a realistic example, Fisher regulators are used. The method is equally applicable to regulators and relief valves produced by other manufacturers, and the regulator and relief valve can be from different manufacturers. A 2 in. size Fisher S21 regulator with a 1 in. diameter orifice has been selected for service as a second-stage propane regulator. Input pressure from the first-stage regulator is 1 psi (69 kpa), and outlet pressure is desired to be 14 in. (3.5 kpa) w.c. An independent pressure relief valve will be used to provide overpressure protection. Size and select the pressure relief valve. Determine the flow capacity of the regulator under emergency conditions. To size the relief valve, the maximum flow possible through the regulator s orifice must be determined. This is a worst-case scenario that assumes the regulator orifice to be wide open and flowing as much gas as the inlet pressure and the internal restrictions of the regulator flow path will allow. From the information provided by the regulator manufacturer, the wide-open flow capacity of this regulator is calculated by using the following equation: Q = P 1(abs) (C g )(1.29) where: Q = flow capacity in standard cubic feet/hour (scfh) of.6 specific gravity natural gas P 1(abs) = absolute inlet pressure in psia [P 1(gauge) ] C g = wide-open gas-sizing coefficient (provided by the manufacturer for each particular model) Note that each regulator manufacturer may probably use different equations or factors, so check the regulator manufacturer s literature for the correct information on wideopen capacity. To solve the equation, find C g for this regulator. Refer to Table S3.1 for this information. The first group of regulators includes the S21 at outlet pressures up to 3 in. (7.5 kpa) w.c., which applies to this calculation. The value for C g for a 1 in. S21 regulator is 7. Therefore, Q = ( psi)(7)(1.29) = 22,34 scfh Note that flow, Q, is in scfh of natural gas even though the example is for propane service. Many regulators are used more extensively for natural gas than for propane. Therefore, the manufacturer s literature is often stated in terms of natural gas. The regulator capacity is left in natural gas units to facilitate reading the performance charts of the relief valves, which are also stated in natural gas units. This method is acceptable since the would-be conversion error cancels itself out. This procedure is a trial-and-error process. Experience can help, but it is first necessary to choose a relief valve that has the desired discharge pressure within its adjustment range and then see whether it has sufficient relieving capacity. First, try a 2 in. size Fisher 289H pressure relief valve. The flow characteristics for this valve in natural gas service are shown in Exhibit S3.1. Because this valve can be used for several start-to-discharge pressures, the proper spring must be selected. Since the second-stage outlet pressure is 14 in. (.5 psi) (3.5 kpa) w.c., the 1 / 2 psi start-to-discharge 28 LP-Gas Code Handbook

3 Supplement 3 Sizing of Independent Pressure Regulators for Large Propane Systems 529 TABLE S3.1 Additional Specifications for All S2 Series Regulators Maximum Operating Inlet Pressure to Wide-Open Port Obtain Optimum C g for Diameter Performance External Type Number (in.) (psig) Relief Sizing S21, S22, or S23; S21P2, S21P3, S22P2, S22P3, S23P2, or S23P3 8 3 / 8 through 3 in. w.c. outlet pressure settings; S21P5, S21P6, S21P8, or 8 1 / 4 S21P S21D With standard 67 Series or optional Type S251 loading regulator 8 1 / 4 With standard 67 Series loading regulator Outlet pressure With optional Type S251 loading regulator 6 With standard 67 Series loading regulator Outlet pressure With optional Type S251 loading regulator 25 With standard 67 Series loading regulator Outlet pressure With optional Type S251 loading regulator 15 S21H or S22H; S21P2, S21P3, S22P2, or S22P3 with 1 to 3 1 / 4 psig 8 1 / 4 outlet pressure settings 8 3 / S21K, All outlet pressure settings 8 1 / 4 S21PK 8 3 / to 5 1 / 2 psig outlet settings to 1 psig outlet settings 25 S23H; S23P2 or S23P3 with 1 to 2 1 / 4 psig outlet pressure settings 8 1 / / S24, S24H, S26, S26H 8 3 / / Source: Fisher Controls Intl, LLC. 28 LP-Gas Code Handbook

4 53 Supplement 3 Sizing of Independent Pressure Regulators for Large Propane Systems psig 1B in. type 289H No vent restriction 1 EXHIBIT S3.1 Flow Curve for 2 in. Size Fisher 289H Pressure Relief Valve. (Courtesy of Fisher Controls Intl, LLC.) Inlet pressure (psig) psig 1B psig 1B psig 1B psig 1B ¹ ₂ psig 1B Specific gravity gas flow rate(thousand cubic feet per hour 14.7 psia at 6 F) Notes: 1 Less than a 5 percent capacity loss can be expected with the vent screen installed on the 2 in. type 289H at maximum flow. 2. When selecting any relief valve for installation downstream of a regulator, the capacity of the relief valve should be compared with the wide-open capacity of the regulator. 3. Bubble point relief setting and spring part number are noted on each curve. 4. To convert to equivalent capacities of other gases, multiply values obtained from curve by the following factors: air.78, propane.628, butane.548, nitrogen pressure curve shown in Exhibit S3.1 cannot be used because that would result in continuous discharge from the relief valve. A 1 psi start-to-discharge pressure can be used because it is between the operating pressure, 1 / 2 psi, and the maximum desired overpressure, 2 psi. A 2 psi start-todischarge pressure would not be appropriate because it is the maximum overpressure. In this case, only one start-todischarge pressure can be selected. It is possible, depending on the equipment and pressures involved, that more than one start-to-discharge pressure will work. In those cases, it may be desirable to select the highest set point that still provides the needed exhaust capacity. This set point reduces the possibility of nonemergency discharge from the relief valve by providing maximum pressure separation between the regulator s operating range and the relief valve s set point From the curve showing a 1 psi start-to-discharge pressure, it can be seen that the curve crosses the 2 psi inlet pressure (the maximum pressure desired) at a flow of about 18, scfh of natural gas. This capacity is less than the capacity of the S21 regulator and cannot be used. Next, a Fisher 289P-6365 pressure relief valve is tried to see whether it has sufficient relief capacity. The manufacturer s performance curves show it to have a 1 psi start- to-discharge setting that produces in excess of 28, scfh of natural gas flow capacity while maintaining less than 2 psi outlet pressure buildup. It would, therefore, be an appropriate choice to provide overpressure protection for the system downstream of the S21 regulator of this example. (See Exhibit S3.2.) Relief (inlet) pressure (psig) Relief performance for 2 in. type 289P Registration and pressure reading taken in 2 in. inlet piping. Body outlet vented directly to atmosphere. Spring range.5 to 2. psig (.3 to.15 bar)with spring 14A9672 X Specific gravity gas flow, 1 scfh (6 F and 14.7 psia) EXHIBIT S3.2 Flow Curve for 2 in. Size Fisher 289P Pressure Relief Valve. (Courtesy of Fisher Controls Intl, LLC.) Relief (inlet) pressure (bar) 28 LP-Gas Code Handbook

5 Supplement 3 Sizing of Independent Pressure Regulators for Large Propane Systems 531 REFERENCE CITED IN SUPPLEMENT 3 1. UL 144, Standard for LP-Gas Regulators, 1999, Underwriters Laboratories Inc., Northbrook, IL. 28 LP-Gas Code Handbook

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