Virtually all process and energy facilities require the routine catching of samples of gas and liquid streams. Generally the samples are taken to a lab for analysis. The information provided can be used to adjust operating conditions or to provide information to customers and suppliers.
The process of catching samples is generally a routine one. There are many types of equipment for catching samples, and many of them incorporate built-in safety features. Nevertheless, the activity can be hazardous for three reasons.
- It creates an opening between the process, which is often at high temperature and pressure, is created.
- Because the activity is carried out so frequently, the likelihood of something going awry can be quite high.
- There is usually a need to flush the sample equipment itself in order to removal residual fluid. This means that there has to be some means of disposing of the flush material.
Guidance and discussion to do with the catching of samples is provided in this Safety Moment.
The following issues should be considered when designing and installing connections.
- Dedicated connections should be provided for sample collection. The use of bleeder and vent valves should be avoided.
- Sample take-off connections should be easily accessible and should be at ground level.
- For liquids proper drainage must be provided below the sample point.
- Sample take-off connections should never be located at dead ends of piping, otherwise the material being sampled may not have the same composition of the process streams themselves.
- If the material to be sampled is very viscous or has a high pour point precautions will have to be taken to prevent it from freezing or setting up.
- The sample line should be as short as possible. If it cannot be made short then a sample loop, as described below, will be needed.
Sample Point Locations
The following suggestions should be considered when deciding where to locate a sample point.
- As far as practicable, sample connections should be grouped together and provided with a sample cabinet and one common drain facility.
- The sampling point should be positioned such that the valves are easy to operate and taking the sample will not impair the safety of personnel or plant or cause environmental impact.
- Good locations for sampling are typically the discharge of pumps and the suction of compressors.
- The sample should be maintained in a single phase at a point where the gas is at least 10°C above dew point or the liquid is at least 10°C below the bubble point.
The following issues should be considered when developing procedures and training for the use of sample valves.
- The operator catching the sample or draining the tank must remain at the location all the time.
- He or she should stand at a location where they can immediately shut off the flow of liquid if a fire or large spill should occur. An open valve should never be left unattended.
- If self-closing valves have been provided these should never be blocked or tied open.
- All open-end connections should be plugged when not in use.
- The amount of sample flush should be minimized and all flush should be routed to an appropriate safe collection system or location.
Sample Valve Loop
If the distance between the body of the material to be sampled and the sample point itself is long, thus requiring a long flush time, a circulation loop should be set up as shown in the sketch.
If a sample loop is required, the following issues should be considered.
- Sample points should have two valves: one at the take-off point from the process pipe and another at the sampling point.
- The block valve at the take-off point should have the same size as a standard drain valve.
- The sampling valve size should be maximum DN 15 and should have good throttling properties.
- Sample outlets for self-igniting or very toxic products should have a self-closing downstream valve.
Samples should be taken from a vertical pipe where possible. If this is not practicable the connections for gaseous samples should be installed at the top of the pipe and at the side of the pipe for liquids. Piping will typically be at or near DN 15.
The procedure for using the sample loop is as follows.
- Normally valves A and B and the sample valve are closed and the control valve is open.
- When a sample is to be taken, A and B are opened for a sufficient period of time to allow fresh material to flow through the sample circuit.
- The control valve, C, will continue to operate but the operators need to be aware that it now has a manual bypass and therefore cannot be relied upon to shut off the flow if needed.
- The sample valve is opened and the sample caught (in a bottle for liquids or a sample bomb for gases).
- Once sufficient material has been captured the sample valve and valves A and B are returned to the closed position.
- It is important not to leave A and B open because otherwise the control valve will not be able to fully stop the flow of fluid during normal operations. Also, once they are closed, if the sample valve is opened by mistake not much liquid or gas will escape.