Generally, on piping >2" NPS, the root valve (the one connected directly to the main pipe line) is a pipe spec valve, attached by means determined by the pipe spec, at the minimum size permitted in the spec. - sometimes 1/2" NPS, sometimes larger. Generally this is a block valve giving a straight open port when open , such as a gate or ball valve. This valve is attached to the pipe by durable means per the pipe spec, so the entire instrument system can be removed/repaired/replaced without de-inventorying the line. Any manifold type block and bleed/bypass/equalization valves are located downstream of this valve (or valves, if used for differential pressure instruments). Generally, these supplementary valves are located at the instrument. Impulse lines of pipe or tubing, if required, connect the root valve to the instrument and/or its block/bleed. Those valves, where required, are often needle valves, and often they are unitized in manifolds. Sometimes they are discrete tubing valves.
Diaphragm isolated instruments when used for level, often require full port block valves of the same size of the diaphragm, ie 2 or 3". That's a huge nuisance on a small tank...but so is a diaphragm that doesn't work because a layer of sludge has built up against it because the diaphragm was served with a line of smaller diameter than the diaphragm and hence wasn't free to drain...Diaphagm seals are also sometimes fitted with a flushing ring with a drain and/or vent/flush port with plugs or with valves, gennerally again being ball or gate valves.
In smaller lines, the piping root valve is often dispensed with. A block/bleed can be mounted directly to the pipe branch. In category D services, sometimes instruments are mounted without valves.
"KP-LOK Electric allows us to do our jobs better. Without it, we couldn't keep up!"
Better Switching power supply