Pressure Control Devices
In this block a number of options are provided for inserting pressure control devices:
OC Orifice Size and P.Drop - When this fitting is selected the program calculates the flow and pressure required at this point such that the water flow at each of the discharges downstream is not less than the minimum and then determines the pressure drop required at this point. The program as well as listing the required pressure drop, also lists the required pressure at the downstream node and the diameter of an orifice plate to achieve the required pressure drop.
An OC cannot be inserted in a pipe with a connection to an input point upstream and downstrem OR if there is a parallel path to any of the nodes downstream
OF Orifice Plate Fixed Size - This is an orifice plate with a nominated size used to control the pressure drop at the point of insertion. The program calculates the pressure drop of the orifice plate and includes this in the pipe pressure loss calculations.
OD Orifice Plate Fixed P.Drop This is an orifice plate with a nominated pressure drop. The program includes this entered value in the pipe pressure loss calculations and also calculates the orifice plate size.
OP Pressure Limited This a pressure control device which limits the pressure at the downstream node (in the direction of flow regardless of which order the nodes are entered) to a nominated value.
An OP cannot be inserted in a pipe with an input point on the upstream or downstream node OR if there is a parallel path to any of the nodes downstream
PZ Zone Pressure - This is a pressure control device controlling the pressure at the top of a zone in accordance with “AS 2118.6-2012 Automatic Fire Sprinkler Systems - Combined Sprinkler and Hydrant Systems in Multistorey Buildings” The entered pressure is the pressure at the downstream end of the pipe and the node numbers of the pipe must be in the direction of flow
OR - Fixed Pressure Ratio Pressure Control device which controls the pressure at the downstream node of a pipe to a fixed percentage of the pressure at the upstream node.
In the one project there can be any number of OF and ODs in any pipe including similar pipes. OC's cannot be entered in series.
The calculation of orifice plate size or pressure drop is based on the following formula:
Qo = Cd x Ao x [ ( 2 x PD / ñ ) / ( 1 - ( do / D ) ^ 4 ) ] ^ 1/2
where
Qo is the volume flow rate through the orifice (or pipe)
Ao is the area of the orifice
PD is the pressure drop through the orifice plate
ñ is the density of the fluid in the pipe
do is the diameter of the orifice
D is the internal diameter of the pipe
Cd is the flow coefficient of the orifice
The flow coefficient used is listed in the table below for varying values of do/D these values being determined from the data presented in the Australian Standard AS2118 Part 1 Automatic Fire Sprinkler Systems (Appendix C). and in the Singapore Standard CP52.
do/D 0.5 0.55 0.74 0.8
Cd 0.61 0.71 0.86 0.95
For values of do/D between 0.5 and 0.8, values of Cd are linearly interpolated.
Note that the data in AS2118 is restricted to pipe internal diameters of between 50 and 200 and do/D ratios between 0.5 and 0.8 mm. Hence the program does not allow entry of fixed orifice plate sizes (OF) for diameters or do/D ratios outside these ranges. OC, OD and OP can be entered for any pipe size but if the pipe diameter is outside the above range, the orifice plate size is not calculated. If the do/D ratio is outside the above range the orifice size is calculated based on extrapolated values of Cd (which may produce inaccurate values) and printed for assistance to users.