An analysis of the conservation of mass and its importance in balancing equations

Published in Applied Physics Research journal Vol.

An analysis of the conservation of mass and its importance in balancing equations

September 25, One of the most important tasks in plant design is the sizing of the process pipework.

An analysis of the conservation of mass and its importance in balancing equations

Sizing your pipework incorrectly will have a major impact on plant capital cost, operating costs and plant reliability. Plant maintenance and energy usage associated with fluid transport add to these costs. As a designer, it is your responsibility, to make the most effective design possible. We need a way of comparing economic performance of piping in order to make informed decisions.

Pipe sizing decisions should be made by comparing capital and operating costs, which necessitates access to up to date cost data, pipe run lengths and plant design requirements such as flows, temperatures and pressures.

Paradoxically in many design projects it is often uneconomic to carry out an economic design of each pipe unless you use the assistance of a software package.

The chart below shows such an approach. Installed costs include material, installation, labor and increase with pipe size. Operating costs, either annualized or over summed over the project lifetime, include energy and plant maintenance and decrease with pipe size.

By adding the two costs we get a minimum exact economic pipe size. There are many computer software applications that help size flow through pipes, but you need to examine the basis for such programs, otherwise significant errors will destroy the validity of the program for this purpose.

For example, the fluid phase state and flow characteristics are most important considerations, would you use the same criteria to size a pipe transporting cold water as a pipe transporting steam condensate? The most beneficial approach is to size the pipework together with other fluid equipment items such as pumps, control valves, heat exchangers etc.

This is feasible only by using specialised software designed for this purpose. As we have stated the most complete method for pipe sizing is based on economic velocity, but it is worth noting that this approach should be used with caution.

There are clearly many situations where pipe sizing to economic criteria should not be used, some examples are; pump suction pipework, fluids containing solids that may settle, two phase flow, non-Newtonian liquids, steam condensate lines and pipes that are operated intermittently.

We should also remember that it is not necessary for pipe sizing to be exact; this is because we will usually select a pipe size based on standard pipe sizes. Since the application of pipe sizing rules can never be exact, engineers often use simpler approaches such as sizing to a given pipe velocity or a specific pressure gradient.

Some of the disadvantages of using the simpler methods are that recommended velocities vary with each fluid and each piping material, meaning that we need to remember and use multiple recommended velocities.

Another disadvantage is that recommended velocities change with time. This reflects the relative large increase in energy costs compared to capital costs over this time period.

Use pipe sizing rules to give an exact calculated size. There are many approaches you can use here such as explained in the literature sources, 1, 2, 3 Select the appropriate pipe size after considering all design factors, fluid characteristics, phase states etc.

Piping Systems FluidFlow is a software product that has been designed to include the above considerations and we will work through two different examples methanol transfer and then a condensate recovery systemto help illustrate the pipe sizing considerations used.

The aim of pressure relief valves is to prevent damage to equipment, prevent injury to personnel and to avoid potential risks to the environment. It is therefore critical that proper relief-valve sizing is carried out to ensure the fluid has sufficient flow area to exit the process thus safeguarding the safe operation of the process plant and equipment.

This is essential to ensure maximum protection in line with the design intent. There are many different ways in which high pressure can be encountered in a system such as, failure of a control valve, external fire, thermal expansion of a liquid, or a reaction which becomes out of control.

Organic Chemistry

Each potential cause of overpressure is generally referred to as a scenario and all potential overpressure scenarios must be identified, characterised as credible or non-credible and documented prior to sizing a relief valve or relief system.

Pressure relief valves have been known to be sized and selected simply by matching the connection size of an available vessel nozzle or the size of the pipeline connection.

Correct relief valve sizing is a complex process and there are many methods and tools available to size such devices. The latter Standard is the most widely used for relief valve sizing.

These standards outline several equations for sizing relief valves for various flow conditions such as liquid flow, gas flow critical and sub-criticalsteam flow and two-phase flow. Before we consider the subject of relief valve sizing any further, we should firstly understand some of the terminology used.

This is the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure. The pressure is the least of the values for the internal or external pressure as determined by the vessel design rules for each element of the vessel using actual nominal thickness, exclusive of additional metal thickness allowed for corrosion and loadings other than pressure.Fundamentals of ED and ion exchange membranes are presented.

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