CPD 13 2015: Introduction to sanitary pipework design

INTRODUCTION

This CPD will outline the key considerations when designing pipework layouts for sanitary systems in accordance with the British and European standard BS EN 12056: “Gravity drainage systems inside buildings”. The precise size and design of a system will depend on factors such as building height and use and the number and type of appliances that discharge into it. However, all systems have key elements in common, including the use of water-sealed traps and a detailed ventilation strategy.

RELEVANT STANDARDS

A number of regulations and standards must be adhered to in the design of a sanitary pipework system, depending on location and scope of work. The current British standard has evolved from the London City Council drainage byelaws dating back to 1875. This was followed by a British standard on sanitary pipework, BS CP 304, in 1953 and 1968, and by BS 5572 in 1978 and 1994. In 2000, BS EN 12056 was introduced; part 2 of the standard relates to sanitary design.

Since 1965, the design of sanitary systems has been controlled nationally by Part H of the Building Regulations: Drainage and waste disposal. Although this gives guidance on system design, it simply states that the system must be adequate. The designer can prove that it is by following a credible design strategy – for example, based on BS EN 12056:2 and the use of products certified by the British Standards Institute (BSI ) or the British Board of Agrément (BBA).

TRAPS

Different countries use different design criteria for sanitary systems, but in the UK they are based on the use of water-sealed traps, which prevent foul air from entering the building. These should be installed at all points of discharge into the system and need to retain a minimum seal of 25mm of water or equivalent. The table below gives minimum trap sizes for common sanitary appliances.

Traps come in various configurations, such as low-level bath and shower traps for fitting in shallow spaces, anti-siphon bottle traps, which can be used under washbasins with a vertical pipe or long run of pipe, and “S” or “P” tubular traps. They should be attached immediately below the outlet, or as close as possible, and should be designed so that they are easily accessible.

PIPEWORK MATERIALS AND JOINTS

A variety of materials are used for soil and waste pipework. They are covered by a number of different standards, as shown in the table below:

Each material is jointed in a different way:

• PVCu soil pipes can either have a ring seal, which means they push-fit together, or they can be solvent welded
• HDPE soil pipes are jointed by electrofusion or butt-welding, which is quick and simple but requires the use of specialist equipment. Push-fit joints are also required to accommodate thermal movement
• ABS and PVCc waste pipes are solvent-weld jointed
• PP acoustic soil and waste pipes can only be jointed using the push-fit method
• Polypropylene waste pipes are push-fit jointed.

Branch pipes serving a single appliance should have at least the same diameter as the appliance trap.

DISCHARGE STACKS

Branch pipes connect to a central stack which discharges to a drain. When designing the stack, a number of factors need to be considered.

• Positive pressure. To eliminate positive pressure, the minimum distance from the base of the stack to the lowest branch connection varies depending on the height of the stack, as the table shows:

The stack base should use two 45˚ bends or a bend with a radius of 200mm or more.

• Stub stacks. Branch pipes can also discharge into an unventilated “stub stack” as long as they are only used to connect ground-floor appliances. The vertical drop to the drain must not exceed 1.5m from a WC and 2.5m from a waste appliance. The head of the drain should be vented to the atmosphere. The maximum loading of a stub stack is 5 litres/second.

• Ventilation. Discharge stacks need to be ventilated in order to prevent the water seal in the trap from being broken by a pressure build up in the system. A secondary ventilation stack is sometimes required to increase stack capacity due to the number of appliances connected. The table below shows the different options depending on the calculated flow rate of the system.

Another option is to specify an aerator, which eliminates the need for secondary venting by breaking the flow on each floor, thereby keeping the pressure difference well within the limit of 3mbar. A 110mm aerator can increase the stack capacity to 7.3l/s.

Branch pipes will not need separate ventilation as long as they do not exceed the length and gradient shown in the following table:

If a branch pipe exceeds these limits, it should be connected via a branch ventilated pipe to external air or a ventilating stack, or an air admittance valve should be used.

An air admittance valve can be used to terminate primary vented discharge stacks, providing the air flow rate of the valve is at least eight times the peak flow rate on the stack.

• Size of stack. In order to work out the correct size of the stack for a specific system, the designer has to add up the number of discharge units and for each one apply a factor depending on the type of use – for example, a house, school or hospital. The procedure and calculation method are explained in Tables 2 and 3 and clause 6.3 of BS EN 12056-2.
• Prevention of cross-flow. A branch pipe should not discharge into the stack in a way that could cause cross-flow into any other pipe. To prevent cross-flow from a large-diameter branch into a smaller connection, the latter should join the stack in one of the following ways:
  • Above the centre line of the larger branch
  • At right angles to the larger branch
  • At least 200mm below a restricted zone beneath the centre line of the larger branch. The depth of this zone is determined by the diameter of the stack: 90mm for an 82mm stack; 110mm for a 110m stack; 250mm for a 160mm stack.
  Specially designed fittings can also alleviate this problem, such as the eight-way collar boss, which has four vertical and four horizontal connections. To link multiple WCs to a soil stack, a manifold system is used, varying the angle of the connector at each WC.
• Offsets. Due to the design of commercial buildings, discharge stacks often need to be offset. When calculating the required flow capacity, the stack cannot be sized as a secondary ventilated system unless vent connections are incorporated on every floor. No connection is permitted within 450mm of the offset.

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