VALVES REPORT Trapped key interlocking: a mechanical permit to work

Key transfer system prevents inadvertent
or unauthorized operation of valves

Alan Ranger
Sigma Controls

Typical pump changeover scheme.

Interlocking is a means of positively controlling both the safety and process of any substance or liquid by a key transfer system which can be coupled to an electrical indicator or PLC.

Use of interlocking is both simple and extremely versatile. Almost any sequence of control and hazard protection can be achieved using a `building block' approach - whether the aim is to provide people or plant protection.

Over 70% of accidents in the oil and gas industry are caused by human error, and account for 90% of the financial costs of accidental loss. Interlocking can minimize the risks associated with many `routine' industry procedures, such as valve servicing, by ensuring that work is carried out in the correct sequence.

A trapped key interlocking system works by ensuring that a control key (or keys) can only be released when a pre-determined condition exists. Removal of the key prevents that condition being reversed, enabling it to be used to gain access to the hazardous area or to activate a further control step.

There is no limit to the number of secondary keys or functions that can be controlled by using exchange units: this also enables retrospective modification of existing systems to be undertaken with ease.

Offshore there are a large number of applications where valve and trapped key interlocking have already proved their worth, from tanker loading bays to pig launcher/receivers and gas sampling points.

Predetermined sequencing

In the oil and gas industry, use of valve interlocks prevents the inadvertent or unauthorized operation of a valve, and by the unique coding of operating keys enforces a predetermined sequence for the operation of multiple valves. It is also possible to mechanically interlock any piece of electrically operated equipment by hard-wiring a switch into the equipment power supply. Typical applications are motorized valves and pumps.

The same key code mechanism can also be used in trapped key bolt type interlocks - supplied with either a single or dual key operation - typically used for restricting access to hazardous areas. Almost any interlocking sequence can be accommodated.

On the safety side, interlocking provides a guarantee that a procedure or system is followed to the letter. On the process side, it is a guarantee that a set procedure of valves - allowing mixing, purging, bypassing or isolating - will be followed through.

All interlock keys may be labeled or tagged to assist the operator to follow the process through. A key storage cabinet in the control room, labeled in the same way as the first and last key in the sequence, will indicate which systems are up and running and which are in maintenance. The key is, in effect, a mechanical permit to work. Such monitoring can be complemented with a PLC.

The ideal situation is to have PLCs combined with interlocks. The interlocks provide the mechanical locking with the monitoring PLC providing a second level of safety. Physical key transfer ensures that the operation is completed in the correct sequence.

Interlocks - both valve and trapped key bolt types - should be easy and quick to install, both to new plant and for retrofitting. With possible exposure to corrosive chemicals, all parts must be of a high-grade stainless steel. The locks should be tamper-proof, weatherproof, maintenance free and requiring no hot work or drilling for installation.

Sigma's own Prosafe valve interlocks are also fire-proof (in accordance with BS6755 Pt 2, 1987) functioning completely after being subjected to a temperature of over 760(C for 30 minutes. Uniquely, these rising-stem valve interlocks also allow `nipping-up'.

As a valve seat wears, the interlock, which is designed to lock in the open or closed position, can still be tightened down without inserting a key, preventing seepage. The valve remains locked, and any attempt to open it results in the handwheel rotating freely.