Systems Of Piping

Piping systems

A ship's machinery space contains hundreds of metres of piping and fittings. The various systems are arranged to carry many different liquids at various temperatures and pressures. The influences of operational and safety requirements, as well as legislation, result in somewhat complicated arrangements of a few basic fittings. Valves, strainers, branch pipes, etc., are examples of fittings which are found in a pipe system.

Pipes

Machinery space pipework is made up of assorted straight lengths and bends joined by flanges with an appropriate gasket or joint between, or very small-bore piping may use compression couplings. The piping material will be chosen to suit the liquid carried and the system conditions. Some examples are given in Table.

Where piping is to be galvanised, the completed pipe with all joints fully welded is to be hot-dipped galvanised. The pipes are supported and held in by hangers or pipe clips in such a way as to minimise vibration. Steam pipes or pipes in systems with considerable temperature variation may be supported on spring hangers which permit a degree of movement. An alternative to spring hangers is the use of expansion loops of piping or an expansion joint.

Must read ➤ Types Of Piping

Table Of Pipework material. 

 

Valves System

Valves are provided in a piping system to regulate or stop the liquid flow. Various types exist with their associated particular function or advantages.

Cock System

A cock is used in small-bore pipework and is joined to adjacent pipework by a compression coupling. A cock can restrict or close an internal passage by moving a central plug, usually by an external lever. An example of a straight-through cock.

Globe valve System

A globe valve has a spherical body enclosing the valve seat and valve disc. Flanges are provided at either side for connecting to adjacent pipework, and internal passages guide the liquid flow through the valve seat. Liquid flow is always arranged to come from below the valve seat so that the upper chamber is not pressurised when the valve is closed.

Read also ➤Pump Definition

A screw lift valve arrangement is shown where the spindle is joined to the valve disc. A gland with appropriate packing surrounds the spindle where it leaves the valve bonnet. The upper part of the spindle is threaded and passes through a similarly threaded bridge piece. A circular handwheel is used to turn the spindle and raise or lower the valve disc. 

The valve disc and seat are a perfect match and may be flat or, more commonly, mitred. The material for both is often provided with a very hard stellite coating. Globe valves also exist in a right-angled form where the inlet and exit flanges are at 90° to each other.

Non-return or check valves are arranged in various pipelines to prevent reverse flow. Where the valve disc is not attached to the spindle it is known as screw-down non-return (SDNR). The valve disc in such a valve must have some form of guide or wings to ensure it can reseat correctly when screwed closed. 

Non-return valves are sometimes arranged without spindles, in which case they are liquid-operated and can not be manually closed. A free lifting valve may be used or a hinged flap.

Gate valve System

A gate valve should be fully open or closed; it is not suitable for flow control. When open it provides a clear full-bore internal passage for the liquid since the valve or gate is raised clear. The spindle is threaded over its lower portion and when turned causes the gate to raise or lower. The gate may be parallel or wedge-shaped in a section fitting against a matching seat. Larger valves have replaceable seat rings and gate facings.

Relief valves System

Excess pressure is avoided in pipe systems by the use of relief valves. The valve disc is held closed by a spring arrangement on the stem. The spring compression can be adjusted to enable the valve to open at the appropriate pressure. Boiler safety valves are a special case of relief valve.

Quick-closing valves

Oil tank suction valves are arranged for rapid closing from a remote point by the use of quick-closing valves. The collapsing of the 'bridge' results in the valve closing quickly under the combined effects of gravity and an internal spring. A manually operated wire or a hydraulic cylinder can be used to collapse the bridge.

Valve chests

Valve chests are a series of valves all built into a single block or manifold. Various arrangements of suction and discharge connections are possible with this assembly.

Other fittings

Mud boxes are fitted into the machinery space bilge suction piping. The mud box is a coarse strainer with a straight tailpipe down to the bilge. To enable the internal perforated plate to be cleaned when necessary, the lid of the mud box is easily removed without disconnecting any pipework.

Suction pipes in tanks should be arranged with a bell mouth or foot. The bell end or foot should provide an inlet area of about one-and-a-half times the pipe area. It should also be a sufficient distance from the bottom plating and nearby structure to provide a free suction area, again about one-and-a-half times the pipe area. 

A steam trap does as its name implies and permits only the passage of condensed steam. It operates automatically and is situated in steam drain lines. Various designs are available utilising mechanical floats which, when floating in condensate, will enable the condensate to discharge. Other designs employ various types of thermostats to operate the valve which discharges the condensate.

 An expansion piece is fitted in a pipeline which is subject to considerable temperature variations. One type consists of a bellows arrangement which will permit movement in several directions and absorb vibration. The fitting must be selected according to the variation in system temperatures and installed to permit the expansion and contraction required in the system.