CADFamily.com: Design Of Firewarter Systems-Part A

Introduction:

This document sets out some guidelines for designing firewater systems using PIPENET Spray/Sprinkler module. It considers a firewater system on a process plant. Typically such systems have deluge systems, Monitors, hydrants, hosereels and may have other items.

One approach which is often followed by experienced fire engineers is the following. This is not by any means the only approach, but it has often been followed successfully by experienced fire engineers. Such engineers typically separate the design of deluge systems from that of firewater ringmains. There are essentially two reasons for this.

This is because the objectives of the calculations for the two requirements are somewhat different. The main objectives are summarised in sections 1.1. and 1.2. below. In view of these differences, in order to optimise the design, it is a good idea to divide the system into deluge system and firewater ringmain before the calculations are performed.

Furthermore, sometimes the design of deluge systems is carried out by a company which is different from the company carrying out the design of firewater ringmains.

In this chapter we take two real firewater systems and show how our tips and guidelines described in chapters 1 and 2 may be implemented in practice.

The systems used in this document are real applications for which PIPENET Spray/Sprinkler module has been used. The networks are relatively complex. For this reason the trainee is encouraged to obtain the data files from Sunrise Systems or an authorised representative.

1.1. Deluge System Calculations Objectives

To check the velocities and keep them below the specified limits. Although firewater systems generally do not suffer from erosion problems because they do not work 24 hours a day, it is still important to keep the velocities below a reasonable limit. This would minimise the frictional losses in the pipes. Normally high velocities arise because the pipe diameter is small. For a given flowrate the frictional loss per unit length would be inversely proportional to the fifth power of the diameter. So one must take care while reducing the pipe sizes with the view of reducing costs.

Achieve good flow balance between the nozzles. Generally a well-designed system will have a low flow imbalance between to nozzles. A high imbalance would generally mean the total flow demanded by the system is higher than necessary. In an extreme case this might even lead to a bigger fire pump.

Optimise the inlet pressure and flowrate. It is better to optimise, rather than minimise. Naturally, minimum frictional pressure loss and hence the minimum inlet pressure could be obtained by maximising the pipe sizes. This could increase the cost of the system and its weight. This is why it is important to work close to the maximum velocity but not exceed it.

1.2. Firewater Ringmains Calculation Objectives

Adequacy of pump performance. The deluge system calculations would show the system requirements by way of pressure and flowrate. We need to make sure that the fire pump arrangement can meet the requirements under a variety of conditions and a variety of demands. The skill is designing the firewater ringmain system well is in making sure that it would work adequately under a wide variety of circumstances.

Ability to generate sufficient pressure at inlet to the deluge systems and other users. It is not just the sizing of the fire pumps which is important. The pipes have to be sized adequately too. They must be small enough to reduce the weight and cost but big enough to offer acceptable pressure drops.

Longest piping route by blocking pipes. Firewater ringmains have isolation valves for maintenance, strainers and so on. It is often necessary to ensure that the system would work adequately even if an isolation valve is left closed or a strainer is blocked.

Integrity of the system if a pipe is broken. If a firewater ringmain system is being analysed this may be important. For example, if the pipework near the helideck system is broken in a platform with several levels, it is possible that the deluge systems at the low levels might still work properly albeit at a reduced efficiency. This is because the elevation difference might produce enough static head.

Different fire scenarios. One of the interesting aspects of the design of a firewater ringmain system is the fact that the same piping system must work under a wide variety of conditions. Clearly this means performing calculations for a wide variety of fire scenarios.

Different fire pump scenarios. Usually the system is designed in such a way that one or more pumps would be required to meet the demand. It is necessary to ensure that this is possible.

It is clear that the objectives of the calculations for deluge systems and those for firewater ringmains are somewhat different. This is why best designs are obtained by separating the deluge system calculations from the ringmain calculations.

1.3. Principles of the Design Procedure

The following phases of hydraulic design are considered in this document. It is important to emphasise that it would not be necessary to go through all these phases for every design. In particular it is quite common for the design engineer to set the pipe sizes (diameters) and skip that phase. It should also be pointed out that the document does not allow for safety margins in the design. For example, where the firewater ringmain and the deluge system meet this document works on the basis that the match between then is exact. In the real design case, it may be wise to allow a slight excess pressure on the firewater ringmain side. This is because the fire pumps may not perform as expected or because during fabrication it may become necessary to re-route the pipes and so on.