It's perhaps not always ncessary, but it is good practice to ground all metallic objects in open air (less important if inside a building, but also a good practice).
May also be mandatory under some local regulations. prex
motori@xcalcsREMOVE.com
It is a common and wise practice to ground all major structures in an industrial facility.....This incldes the building steel, piping, tanks, heat exchangers and pressure vessels. Plant safety is maintained by control of stray currents through good grounding practices.
Grounding is discussed in some of the electrical texts available and, I recall, NFPA -77 bulletin.
Having grounding pads added to components costs almost nothing.
In flammable liquid service, it is necessary to properly ground the vessel (along with other precautions) to prevent the acuumulation of static charges that could result in a spark igniting flammable vapors. NFPA-77 and API RP2003 are the two most commonly referenced standards with respect to grounding.
Electrical grounding is necessary to prevent electrical shock hazard to personnel, electrical short hazard when not if in contact with electrical energy and static energy sources , lightning protection, and explosion hazard from combustionable products like dust and airborne solid particles and combustionable gases. A good example of an almost impossible item to properly ground is a mobile home.
The question is not "should it be grounded". The question is "How to properly ground" considering that most all items have multiple grounding concerns which can not be simultaneously resolved. For example, electrical systems have a common, an electrical ground, and a structural ground which are not necessarily equal in electrical potential.
Here's an odd side effect of not grounding your pressure vessel: your thermocouple reading may read strangely, especially if you are using a grounded T/C. Recall that a thermocouple is really just a small circuit that changes its amperage as the temperature changes. An ungrounded pressure vessel with a build up of static electricity can read differently than a grounded vessel.