Evaluate pressure drop in vacuum systems

A simple analysis of plant vacuum systems can reduce utility expenses.

By Dan Bott

Organizations typically take great care in the selection of process vacuum pumps. They evaluate and select vacuum pumps based on factors such as performance, energy utilization, size, sound level, and price. However, the components installed between the process vacuum pumps and the production machinery they serve is vitally important to the proper functioning of the system. As it stands, one of the most neglected aspects of vacuum supply is the distribution system. The vacuum distribution system is important because of problems that pressure loss creates. Vacuum pumps must operate at an elevated level of vacuum to compensate for excessive pressure drop. This costs energy dollars and magnifies leak problems. Simple and cost effective measures alleviate these problems. These measures raise the operating efficiency of the system.

Pressure drop

What is pressure drop in a vacuum system? The idea of pressure drop in vacuum systems is similar in concept to pressure drop in plant compressed air systems. Pressure drop is the difference in operating pressure from the supply point to the use point. In compressed air systems, pressure drop is measured in psid, which means pounds per square inch differential. Pressure drop can be used as a measure of pressure drop in vacuum systems as well, but it is much more common to use inches of mercury as unit of measure. Table 1 illustrates some of the common pressure units used in vacuum and how the scales compare with one another. Note that the top of the table represents atmospheric pressure and the bottom of the table represents perfect vacuum. Also, note that there are two scales that use inches of mercury.

Finding the pressure drop in a vacuum system is a simple procedure. First, measure the vacuum at the inlet to the vacuum pump using a reasonably accurate vacuum gauge. Then, using the same gauge, measure the vacuum at the point of use or as close to the point of use as possible. It is acceptable to use two separate gauges, but use calibrated gauges or gauges that display similar values for the levels of vacuum found in the system. The difference between the vacuum levels at the pump inlet and the point of use is the total system pressure drop.

Depending on the type of system, the product manufactured, and the operating vacuum level, the average system pressure drop varies from a fraction of an inch of mercury to 15 inches of mercury. Higher pressure drop means higher operating costs.

Operating demand created by pressure drop

To illustrate the artificial demand created by pressure drop in a vacuum system, consider an average process that requires a volumetric flow of 100 actual cubic feet per minute at 20 inches of mercury vacuum. This example can be scaled up or down to fit any particular application. As system pressure drop increases, there is need for higher vacuum at the pumping system to compensate for the loss. This may not be a problem in moderate levels. A particular vacuum pump may have extra capacity available and some vacuum pump technologies actually use less brake horsepower as vacuum increases. In more severe cases, however, extra capacity will have to be added to the system to account for the greater volume of air. Additional capacity is required because, in general, as vacuum level increases, air entering the system expands in proportion to the vacuum level. The higher the vacuum, the greater the expansion. To attain the desired vacuum and overcome this expansion, more volumetric capacity is needed. At some point, the installed vacuum pump will not be able to keep up with the required expansion of air.

The amount of additional capacity required depends upon the starting vacuum level and the pressure drop. Table 2 illustrates how this phenomenon affects our example system. It shows how pressure drop can add significantly to the number of vacuum pumps required to run a production process. A pressure drop of 3 inches of mercury adds 42 percent to the required production acfm flow. It is not difficult to see that reducing system pressure drop reduces energy costs by reducing the number of vacuum pumps that must be on line. Operating fewer vacuum pumps not only saves energy dollars, it also reduces plant maintenance costs. Non-tangible benefits to fewer on-line pumps are lower noise level, less mist carryover, and lower ambient heat loads. Eliminating off-line pumps can free up usable floor space.

Leaks

Operation of a vacuum system at higher levels also affects the volumetric flow of air leaking into the system. Air entering a vacuum system through leaks adds to the production volume demand and must be treated as if it were production demand. As the vacuum level increases, the affect of leaks on the system increases as well. If our example system has a leak flow rate 6 acfm at 20 inches of mercury vacuum, then operation of the same system at 25 inches of mercury vacuum doubles the leak rate. Even though the percentage of total flow remains the same, it is still an additional load on the vacuum pumps. Also, depending upon the design characteristics of the distribution system, the increased differential pressure from operation at higher vacuum levels may open more leaks.

Reduce the pressure loss

After measuring total system pressure drop, determine which components are adding the most restriction. Review each component individually and then rank them against each other to implement a worst first repair program. To check an individual component, tap into its inlet and discharge and measure the pressure drop. Measure and record the pressure drop for each component from the vacuum pump to the point of use. Establish running logbooks for some items, like filters with replaceable elements, to determine element change-out intervals. The following list contains areas that should be checked or reviewed for each in-house vacuum system.

Piping diameter is the single biggest problem with vacuum systems. The combination of restrictive pipe diameter and lengthy piping runs creates significant pressure drop. As a rule of thumb on single vacuum pump applications, maintain the diameter of the vacuum pump inlet as far into the process as possible. Pipe with smooth interior walls is superior to rough walled pipe. Piping run should be as kept short and straight. Minimize the use of elbows and, where they are necessary, use large radius elbows in place of 90-degree turns. Complete a full analysis on multiple vacuum pump applications to determine the optimum pipe diameter.

Inspect isolation valves and check valves to ensure they are full-port and they match the diameter of the system piping. The port diameters of standard ball valves are too restrictive for vacuum applications. Full-port ball, gate, or butterfly valves provide excellent flow characteristics and very little restriction. Check valves can also be a source of restriction in vacuum piping systems. Repair immediately check valves that become lodged or fail to open completely.

Most vacuum pump technologies require inlet filters to remove particulates from the incoming air stream. Filter element loading increases pressure loss and can be easily avoided with proper preventative maintenance. Improperly sized filters with small port diameters can also be a major source of restriction. Check with the filter manufacturer to ensure proper sizing and installation.

At times it is necessary to use receivers and separators to remove liquids from the vacuum air stream upstream of the vacuum pump inlet. It is important to have the correct type, configuration, and porting on receivers and separators to ensure adequate liquid separation and low pressure drop. Many separators have minimum and maximum velocity requirements for optimum separation efficiency. It is important to follow these guidelines to provide maximum protection for the vacuum pump.

Production machinery sometimes accounts for the majority of system pressure drop. Conventional thinking, however, does not allow for changes to the internal plumbing of production machinery. Given that the thought process for production machinery design usually does not take into account the energy usage of vacuum supply, it is worthwhile looking into what changes can be made that improve flow and not sacrifice production efficiency. Sometimes, improvements can be as simple as enlarging the internal diameter of supply tubing.

The vacuum pump controls on some vacuum pump technologies automatically regulate the system vacuum level within a preset range. These mechanisms are sometimes set incorrectly or are out of adjustment. Improper functioning of vacuum pump controls chokes off the airflow to the pump and appears to be a plumbing problem even if the rest of the system is functioning optimally. Only qualified service personnel should adjust vacuum pump inlet controls.

No vacuum system evaluation is complete without a leak check. Leak checks are important in some facilities because considerable horsepower is used just to overcome the system leak rate. There are several techniques commonly used for detecting leaks in vacuum systems. Two common methods are ultrasonic detection and tracer gas detection. Both methods are suitable for production vacuum systems.

Results

In many cases, adding vacuum pumps to overcome system pressure drop solves distribution problems. A program that identifies and corrects the significant causes of pressure drop has the potential to forestall or completely eliminate the need for new vacuum equipment. Of course, it is not practical or economically feasible to eliminate all pressure drop from a vacuum system. However, it is possible to eliminate the worst pressure drops so that the tradeoff between operating costs and costs for distribution changes is favorable.

A proactive program assists in taking vacuum pump horsepower off-line. As an example of the amount of savings to be realized for reducing operating horsepower, one 40-horsepower vacuum pump taken off-line results in a yearly savings of $16,845 at $0.06/KWH. This is a significant sum considering the nominal investment in time and plumbing changes.

The other advantage of a proactive approach is an increase in the quality of vacuum supply to end-use points. Once evaluation and repair programs are complete, vacuum distribution systems that had increasing demands placed upon them over time or that were marginally sized to begin with will not be as susceptible to fluctuations in production vacuum load. This results in more production uptime, faster cycles, better-formed products, and increased holding force. In other words, the system has greater efficiency.

Resist the tendency to add horsepower to solve vacuum supply problems. Before purchasing additional vacuum equipment or adding on-line horsepower to solve production vacuum problems, evaluate the vacuum distribution system for excessive pressure drop. It is an effective approach for both cost reduction and cost avoidance.

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