Connecting Utility Lines to Process Lines

Compliance with appropriate national regulations should do the trick.
Double check valves, breaker tanks, air gaps etc would be a good idea.

I expect others will be more specific.
Del

Which particular utility fluid?

Which process fluid?

If used regularly and allowed by local legislation or site standards then double block and bleed (two valves with a spool piece between them that has a drain valve on it) would be appropriate with the addition of a non return valve (which is an added precaution against backflow of process but is never to be relied upon as isolation).

Double valves and spade or spectacle blind - again with non-return valve.

Furnishing a PID is required for proper comment. Even pumps may get affected

The problem is not always so easy to prevent in a large plant. I have had experience with one paper mill where they inadvertently had some process chemicals back feed into the potable water system. No one was hurt or injured, but 10 years later they still supply bottled water for drinking and warning signs at all taps cautioning the water may not be fit to drink. They are concerned about another snafu or some other back feed problem they have not been able to find.

Best course is to establish proper back feed prevention from the get go, and be sure potable water systems are as isolated as possible from process, especially depending on what the contaminants may be.

(And never drink from a gargen hose, or eat yellow snow.)

A backflow preventer. There are several types, the simplest being an air gap. Check-out http://abpa.org/faq.htm

The simple answer to your question is a double block and bleed in which there are two automatic valves separated by a spool piece which is fitted with an automatic vent valve.

However, it is a little more complicated that that. This arrangement must also have two pressure transmitters, one on the process side and one on the utility side of the main valves, and a control circuit.

The control circuit is set-up so that the utility side pressure must exceed the process side pressure by a fixed amount (or percent) in order to open the main control valves and close the bleed valve. When the process side pressure increases enough to reach the utility side pressure limit, or the utility pressure fall enough to reach the process pressure limit, then both the utility and process block valves close and the bleed valve opens. Normally the process side pressure limit is set at 10% below the utility side pressure; and the utility side limit is set so that if the utility side pressure falls to within 10% of the process side pressure the interlock is activated.

Of course the design must also be "fail safe" meaning that motive force and transmitted signal are required to keep the valves in their normal running position. Loss of motive force or loss of transmitted signal (from the pressure transmitters) must result in activation of the interlock. Normally this is accomplished by using spring loaded valves where the bleed valve is "fail open" and the process and utility valves are "fail closed" design. Another critical "final fail" safe measure is that the system must not reset itself without having an operator make a field inspection and physically reset the interlock circuits. This can be accomplished by using "drop out, latching relays" in which the relays must be re-latched in the field before the interlock circuit can be activated and the valves returned to the normal running position.

When the system is returned to service following an interlock activation the spool piece must be purged with utility material to prevent process side contamination with whatever is in the spool piece at the time. For example if an inert gas utility is feeding into an oxygen sensitive process then following an interlock the spool piece will contain air which must be removed before returning the system to service.

This arrangement will provide adequate protection for both liquid and vapor systems. However, one must choose the two limits such that there in no utility contamination of the utility as a result of flow eddies or molecular diffusion form the process side to the utility side.

One last word of caution: One must never rely on check valves to prevent utility supply contamination by process material. Check valves always leak when they are supposed to hold and always hold when they are supposed to leak.

I have successfully used the above design concepts in both liquid and vapor systems handling hazardous materials at pressures between atmospheric and 2,500 psig. I hope this helps.

Either put a spectacle blind in the line, slip blind or a removable spool peice, you should also have a checkvalve in the supply of the utility line to the process. Is this line going to be used during normal operation or just for cleanup purposes? If for cleanup either blind the line or remove the spool while not in use. That has been the practice I have seen used. If for normal operation insure that checkvalve is in there. I personally like the use of utility hoses for non routine operation.