PG-69.2.3 Coefficient of Discharge Method. A coefficient of discharge for the design, K, may be established for a specific valve design according to the following procedure:
(a) For each design, the pressure relief valve manufacturer shall submit for test at least three valves for each of three different sizes (a total of nine valves). Each valve of a given size shall be set at a different pressure, covering the range of pressures for which the valve will be used or the range available at the facility where the tests are conducted.
For each valve design where the coefficient of discharge has been determined that is intended to be restricted in lift, the Manufacturer shall have capacity tests conducted on three valves of different sizes. Each size valve shall be tested for capacity at the minimum lift for which certification is required, and at two intermediate lift points between the full rated lift and minimum lift certification points. Each of the three test valves shall be set at a different pressure.
For each valve tested, it shall be verified that actual measured capacity at restricted lift will equal or exceed the rated capacity at full rated lift multiplied by the ratio of measured restricted lift to full rated lift.
(b) Tests shall be made on each pressure relief valve to determine its lift at capacity, popping, and blowdown pressures, and actual relieving capacity. An individual coefficient, K
D, shall be established for each valve as follows:
Where actual flow is determined by test and theoretical flow, WT is calculated by one of the following equations:
For tests with dry saturated steam
For 45 deg seat
(U.S. Customary Units)
WT = 51.5 x pDLP x 0.707
(SI Units)
WT = 5.25 x pDLP x 0.707
For flat seat
(U.S. Customary Units)
WT = 51.5 x pDLP
(SI Units)
WT = 5.25 x pDLP
For nozzle
(U.S. Customary Units)
WT = 51.5 AP
(SI Units)
WT = 5.25 AP
For tests with water
For 45 deg seat
(U.S. Customary Units)
(SI Units)
For flat seat
(U.S. Customary Units)
(SI Units)
For nozzle
(U.S. Customary Units)
(SI Units)
where
A = nozzle throat area, in.2 (mm2)
D = seat diameter, in. (mm)
L = lift at pressure P, in. (mm)
P = (1.03 X set pressure + 14.7), psia, or
= (set pressure + 2 + 14.7), psia, whichever is greater
= (1.03 X set pressure + 0.101), MPa, or
= (set pressure + 0.014 + 0.101), MPa, whichever is greater
P d = pressure at discharge of the valve, psia (MPa)
WT = theoretical flow, lb/hr (kg/hr)
w = specific weight of water at inlet conditions, lb/ft3 (kg/m3)
To convert lb/hr of water to gal/min of water, multiply the capacity in lb/hr by 1/500, To convert kg/hr of water to liter/min of water, multiply the capacity in liter/min by 1/60.
The average of the coefficients KD of the nine tests required shall be multiplied by 0.90, and this product shall be taken as the coefficient K of that design. All individual coefficients of discharge, KD, shall fall within a range of ±5% of the average coefficient found. If a valve fails to meet this requirement, the Authorized Observer shall require two additional valves to be tested as replacements for each valve having an individual coefficient, KD, outside the ±5% range, with a limit of four additional valves. Failure of a coefficient, KD, to fall within ±5% of the new average value, excluding the replaced valve(s), shall be cause to refuse certification of that particular valve design.
The rated relieving capacity of all sizes and set pressures of a given design, for which K has been established under the provision of this paragraph, shall be determined by the equation:
W ≤ WT x K
where
K = coefficient of discharge for the design
W = rated relieving capacity, lb/hr (kg/hr)
WT = theoretical flow, defined by the same equation used to determine KD, lb/hr (kg/hr)
The coefficient of discharge for the design shall be not greater than 0.878 (the product of 0.9 x 0.975). The coefficient shall not be applied to valves whose beta ratio (ratio of valve throat to inlet diameter) lies outside the range of 0.15 to 0.75, unless tests have demonstrated that the individual coefficient of discharge, KD, for valves at the extreme ends of a larger range, is within ±5% of the average coefficient, K
D.
For designs where the lift is used to determine the flow area, all valves shall have the same nominal lift to seat diameter ratio (L/D).
For pressures over 1,500 psig (10.3 MPa) and up to 3,200 psig (22.1 MPa), the value of W shall be multiplied by the correction factor:
(U.S. Customary Units)
0.1906P - 1,000
0.2292P - 1,061
(SI Units)
27.6P - 1 000
33.2P - 1 061
For pressures over 3,200 psig (22.1 MPa), the value of W shall be multiplied by the appropriate supercritical correction factor, Ksc, from Table PG-69.2.3.
PG-69.3 If a manufacturer wishes to apply the Code symbol to a power-actuated pressure relieving valve under PG-67.4.1, one valve of each combination of inlet pipe size and orifice size to be used with that inlet pipe size shall be tested. The valve shall be capacity tested at four different pressures approximately covering the range of the certified test facility on which the tests are conducted. The capacities, as determined by these four tests, shall be plotted against the absolute flow test pressure and a line drawn through these four test points. All points must lie within ±5% in capacity value of the plotted line and must pass through 0-0. From the plotted line, the slope of the line dW/dP shall be determined and a factor of (0.90/51.45) x (dW/dP) shall be applied to capacity computations in the supercritical region at elevated pressures by means of the isentropic flow equation.
(U.S. Customary Units)
(SI Units)
where
dW/dP = rate of change of measured capacity with respect to absolute pressure
P = absolute inlet pressure, psia (MPa)
v = inlet specific volume, ft3/lb (m3/kg)
W = capacity, lb of steam/hr (kg/hr)
NOTES: The constant 1,135.8 is based on a g factor of 1.30, which is accurate for superheated steam at temperature above approximately 800
°F (430°C). In interest of accuracy, other methods of capacity computations must be used at temperatures below 800°F (430
°C) at supercritical pressures.
PG-69.4 Power-actuated pressure relieving valves, having capacities certified in accordance with the provision of PG-69.3 and computed in accordance with the formula contained therein, shall be marked as required by PG-110 with the computed capacity, corresponding to 3% above the full load operating pressure and temperature conditions at the valve inlet when the valve is operated by the controller, and they shall also be stamped with the set pressure of the controller. When the valve is marked as required by this paragraph, it shall be the guarantee by the manufacturer that the valve also conforms to the details of construction herein specified.
PG-69.6 When changes are made in the design of a safety or safety relief valve in such a manner as to affect the flow path, lift, or performance characteristics of the valve, new tests in accordance with this Section shall be performed.
PG-70 CAPACITY OF PRESSURE RELIEF VALVES
PG-70.1 Subject to the minimum number required by PG-67.1, the number of pressure relief valves required shall be determined on the basis of the maximum designed steaming capacity, as determined by the boiler Manufacturer, and the relieving capacity marked on the valves by the manufacturer.
PG-71 MOUNTING OF PRESSURE RELIEF VALVES
PG-71.1 When two or more pressure relief valves are used on a boiler, they may be mounted either separately or as twin valves made by placing individual valves on Y-bases, or duplex valves having two valves in the same body casing. Twin valves made by placing individual valves on Y-bases, or duplex valves having two valves in the same body, shall be of approximately equal capacity.
When not more than two valves of different sizes are mounted singly the relieving capacity of the smaller valve shall be not less than 50% of that of the larger valve.
PG-71.2 The pressure relief valve or valves shall be connected to the boiler independent of any other connection, and attached as close as possible to the boiler or the normal steam flow path, without any unnecessary intervening pipe or fitting. Such intervening pipe or fitting shall be not longer than the face-to-face dimension of the corresponding tee fitting of the same diameter and pressure under the applicable ASME Standard listed in PG-42 and shall also comply with PG-8 and PG-39. Every pressure relief valve shall be connected so as to stand in an upright position, with spindle vertical. On high-temperature water boilers of the watertube forced-circulation type, the valve shall be located at the boiler outlet.
