Process vessels, including tanks, are some of the most basic components of a process but present some interesting challenges when you need to prepare a specification.  This is because process vessels can perform a wide variety of functions in every plant.  The key is to think through your requirements before approaching the market.

Defining Process Parameters

The most basic question is how your vessel will be used.  Is it intended for mixing or storing?  A mixing vessel will require a more robust construction and different materials of construction than a vessel intended for product or raw material storage.  You will also need to determine whether the vessel requires heating ro cooling or will it operate at ambient temperature.

Material Compatibility

What goes into your process vessel has a large effect on its construction, configuration and final cost.  It is critical to list every possible chemical that will be used in the tank.  This is particularly true for corrosives and solvents that will dictate the materials of construction for the vessel, instruments, nozzles, gaskets and piping.  It is a good idea to use one of the available Chemical Compatibility Databases to guide you in your selection of materials for the vessel.  There are many available, but the Cole Palmer Chemical Resistance Database (link) is extremely well organized and useful.

Operating Conditions

The first operating condition for tanks will be volume.  You will need to determine the working volume for your tank and then oversize the volume depending on the use you have selected in Process Parameters.  A vessel used for mixing, will most likely require a larger “safe volume” to allow for changes in fluid level during agitation and possible increases in the liquids volume during agitation.  Storage vessels require less “operating margin” but may be oversized to allow for operating conveniences.

You will also need to determine the pressure and temperature ranges that your tank will need to operate.  This will affect the materials of construction, the instruments and many of the ancillary components of the vessel.

Once you have gathered this information, developing a specification document will guide you when working with equipments reps and consulting engineers.  A typical Specification Document for a process vessel would contain the following information:

1. Materials of Construction
2. Required Pressure and Temperature
3. Working Volume
4. Total Volume
5. Geometry
6. Nozzles
7. Accessories

Having this information available in one place will reduce the risk of leaving out a critical process component and allowing you to specify the required vessel more clearly.

When something goes wrong with a piece of equipment in your plant, it’s quite simple to replace it when the equipment is fairly current and readily available in the marketplace.  All that’s needed is a model number and a purchase order.  But what happens if this is not the case?  There are many instances where a new piece of equipment is required because the piece we are replacing is extremely old. In addition, there are many circumstances where a change in the process or an upgrade requires a new piece of equipment to be specified.

Whether you handle the specification process yourself, use in-company resource or call in an engineering consultant, you will have an easier time if arm yourself with information and complete some preparatory thinking before you order your equipment.  If you follow these simple steps, you will be more likely to specify and purchase the correct piece of equipment.

First – Know Your Process Parameters.

It is worthwhile to review exactly how the piece of equipment is used in your plant.  Write up a short description that will become the basis of your specification.  What is this piece of equipment used for?  What specific operations is it called on to perform?   It is important to write this description clearly because the equipment rep or consulting engineer may know the piece of equipment, buy they will not know or understand your process.

Second – What Will Come In Contact With the Equipment?

Many plants run multiple products and multiple batches.  It is critical to determine every one of your raw materials or product components that will come in contact with the piece of equipment.  If you are using corrosives and solvents, this becomes particularly important because it will allow the equipment rep of consulting engineer to properly select compatible materials.  It is also important to be through and list every possible chemical that will come in contact with your equipment.  We have seen more than one new piece of equipment destroyed over the one corrosive that was left off of the list!

Third – Think Through the Operating Requirements.

What are the operating requirements that your process will impose on this new piece of equipment.  Are there required volumes, throughput, temperature ranges etc.  This is a good time to talk to your operators and ask these questions.  It si not uncommon to have the “actual” operating requirements different from those on file in the facilities or engineering department!

Once you have assembled this information, you should collect it in a specification format.  This can be a simple narrative document as long as it clearly covers the three areas above.  This document can then be used with your equipment rep or consulting engineer.