Computer Speeds System Design
While the designer’s judgment never will be eliminated, the computer has taken over the calculation of flow rates, pressure losses and other variables and has provided a new dimension of flexibility and accuracy in designing dry chemical fire protection systems.
Three relationships that must be taken into consideration in designing a system are pressure in the storage vessel, pressure loss in the piping elbows and tees and pressure at the nozzle. Mathematically, this is expressed by the formula:
Outlet pressure = pipe loss + nozzle pressure.
First, a rate is assumed. The nozzle pressure, piping loss and outlet pressure are then calculated. If the system does not balance, then a new rate is assumed and the calculations are repeated until the system balances.
Not infrequently, it becomes apparent that the system never will balance with the proposed equipment, and a larger unit may be needed to increase the outlet pressure. Then additional calculations must be made.
The Ansul Company has developed a computer program which will completely analyze a dry chemical system in 5 to 9 seconds. The information required for computer application includes: the length of pipe, number of elbows and tees, type and number of nozzles, unit size and type of chemical to be used.
This data is punched on a standard 88-column card and placed with a computer call card which locates the program on a tape. The computer then sizes the piping according to Underwriters’ Laboratories requirements for the flow rate in various piping runs.
Next, it analyzes the pressure relationships between the nozzle, piping, and outlet pressures, and determines flow rate which will balance the systems. Often, as many as 25 flow rates are tried, involving as many as 300 individual calculations. If the system cannot balance, the computer gives the reason.
In addition to the calculations, the computer can also prepare a bill of materials.
Light water systems
The computer design method has proven to be the only expedient way to design a Light Water subsurface injection system for fuel storage tanks. Here, the problems of applying the agent are very complex—some of the balancing factors being the specific gravity of the fuel, its depth, type and quantity, the gpm delivery rate, lengths of pipe, number of elbows and tees, and the metering of nitrogen used to produce the aerated solution.
The design is further complicated by the fact that the solution is delivered as a compressible liquid. Thus, the friction varies as the pressure varies.
Design flexibility attained
Since the designer and the user often have different viewpoints on a system, the increased flexibility provided by a computer enables the two to work together more effectively to achieve an optimum design.
For example, initial analysis of a proposed system might call for the unit to be placed 150 feet away from the tank. The piping is perceived to contain four 90-degree elbows, three 45-degree elbows, and one swing check valve. The computer analysis might show that this requires 100 feet of 2inch pipe and 50 feet of 2M-inch pipe. But this is only one viewpoint on how the system may be installed.
We have found that the best approach is to compute five or six possible runs of pipe so this offers the customer a greater freedom of choice when analyzing the system. Since the normal calculation time requires only 45 seconds, the additional time is well justified.