one for the locked closed position. During an operating procedure, only one key at a time is free. This key only fits in the interlock belonging to the valve that is to be operated next in the operating procedure. All keys are uniquely coded to avoid the possibility that valves can be unlocked at an inappropriate time. Nowadays intelligent interlocking solutions enable the integration of field devices like pressure or H2S meters in a valve interlocking sequence. This increases safety by integrating operator procedures with DCS and SIS safety systems. Types of Pigs Gauging, separation and cleaning pigs A "pig" is a tool that is sent down a pipeline and propelled by the pressure of the product flow in the pipeline itself. There are four main uses for pigs: Physical separation between different fluids flowing through the pipeline Internal cleaning of pipelines Inspection of the condition of pipeline walls (also known as an Inline Inspection (ILI) tool) Capturing and recording geometric information relating to pipelines (e.g., size, position). One of the most common and versatile is the foam pig which is cut or poured out of open cell polyurethane foam into the shape of a bullet and is driven through pipelines for many reasons such as to prove the inner diameter of, clean, de-water, or dry out the line. There are several types of pigs for cleaning in various densities from foam and in special applications up to . Some have tungsten studs or abrasive wire mesh on the outside to cut rust, scale, or paraffin wax deposits off the inside of the pipe. Other types are fully or criss-cross coated in urethane, or there are bare polyurethane foam pigs with a urethane coating just on the rear to seal and assist in driving the pig. There are also fully molded urethane pigs used for liquid removal or batching several different products in one line. Inline inspection pigs use various methods for inspecting a pipeline. A sizing pig uses one (or more) notched round metal plates as gauges. The notches allow different parts of the plate to bend when a bore restriction is encountered. More complex systems exist for inspecting various aspects of the pipeline. Intelligent pig Intelligent pigs are used to inspect the pipeline with sensors and record the data for later analysis. These pigs use technologies such as magnetic flux leakage (MFL) and ultrasound to inspect the pipeline.