The proper sizing and selection of your changeover manifold will be key factors in determining the system’s performance for its entire service life.
Many applications require a continuous supply of gas to the process. Stopping the flow of gas during operations to replace empty cylinders is not an option. The laser cutting operation is a perfect example of an application that may operate on a single eight-hour shift basis but requires continuous flow throughout the operating period. Stopping production to replace empty cylinders can be costly – time is money.
There are many other applications where the flow of gas must be maintained 24/7. Helium for gas chromatographs and carbon dioxide for incubators are two simple, but good examples where loss of gas flow can have very unwelcome results.
These applications require continuous flow for long periods of time not only during working hours, but when the system is unattended in the evening and during weekends and holidays. Running a gas chromatograph out of helium carrier gas can result in costly repairs and days without analytical results. An incubator without carbon dioxide can result in destroyed samples and the loss of years of research.
What is a changeover manifold?
A changeover manifold is a system of valves and pressure regulators that delivers gas to a process without gas flow interruption. Most changeovers consist of a regulator scheme that reduces the pressure in two stages to achieve a constant outlet pressure.
Users should be aware that there are systems offered that do not provide two stages of pressure reduction, and that the resultant outlet pressure will fluctuate considerably as the system operates, thus requiring the addition of a line regulator down stream from the changeover to provide a consistent pressure to the process.
Selection criteria for changeover manifolds
Each application has a different set of operating parameters that must be evaluated and satisfied. Let’s take a look at six key parameters that users need to understand when selecting a changeover manifold.
1. Automatic or semi-automatic? What’s the difference?
Some suppliers use semi-automatic and automatic interchangeably when describing a changeover manifold. In fact, they are two distinctly different systems.
A semi-automatic changeover normally operates by opposing pressure differential. It switches from the “in-service” side to the “reserve” side automatically, but requires an action by the operator to switch it back from the new “in-service” side to the “reserve side.”
Figure 1. Typical semi-automatic changeover. The lower regulator controls the changeover switch form side to side by virtue of the 180? knob flip. The upper regulator delivers constant pressure to the process.
Typically this is accomplished by flipping a knob, a lever, or operating a series of valves after replacing the empty cylinders. The unit in Figure 1 is a typical semi-automatic changeover.
An automatic changeover manifold functions electronically. The only action required by an operator for this unit to reverse the changeover is to replace the empty cylinders and to re-pressurize the depleted side. The unit in Figure 2 is a typical automatic changeover.
2. High purity or general purpose?
A changeover designed for use with high purity gases uses regulators that have a small internal volume, smoothly machined internal surfaces, and stainless steel diaphragms. The lines are metal pipe threaded fittings sealed with Teflon® tape or metal tubing connected with compression fittings.
A general purpose changeover may use lower quality regulators with elastomer diaphragms and have rubber tubing lines clamped to hose barbs.
3. The gas supply source is important.
The gas supply source to a changeover may include any combination of the following:
- A standard high pressure cylinder, such as nitrogen or helium.
- A cylinder of liquefied gas, such as carbon dioxide.
- A six pack, 12 pack, or manifold of cylinders
- A cryogenic container of argon, nitrogen, oxygen, or carbon dioxide.
- A tube trailer.
- A bulk storage tank.
While you have all of the above choices and perhaps others, your choice of gas source drives your choice of changeover manifolds. If high pressure cylinders are always to be your source, you have the choice of virtually any automatic or semi-automatic changeover system. Substitute a cryogenic container on one side with a high pressure source on the other side and your choices narrow depending on the operating parameters of delivery pressure and flow.
Figure 2. A typical automatic changeover manifold requiring only the replacement of empty cylinders to effect the switching from side to side.
Automatic changeovers like the Auto-Logic® II pictured in Figure 2 allow the user to start with high pressure cylinders on both sides then expand to a cryogenic source on one side and a high pressure source on the other side. If even higher consumption rates become required, users can easily switch to cryogenic containers on both sides. This built-in “system growth” allows the user to expand without having to purchase and install a new changeover unit.
4. Assess delivery pressure requirements.
This sounds like a basic concept, but it is one that must not be overlooked to avoid some common pitfalls. If the changeover manifold is feeding the process directly, then the answer is straightforward – the delivery pressure is the same as the one required by the process.
If the changeover manifold is feeding an application some distance from its physical location, the answer may not be so obvious. Consideration may have to be given to providing a higher outlet pressure at the changeover to account for piping pressure drops and multiple outlet locations and then installing a line regulator at the point of use to reduce the pressure to the local desired value.
A similar consideration must be made if the changeover manifold is feeding multiple operations. For example, one changeover manifold feeding many gas chromatographs. In this instance, a line regulator should be installed at each chromatograph to ensure that the delivery pressure to the chromatograph is stable and does not change when pressure is changed at one of the other units.
5. What is the maximum required flow?
This is often the most difficult parameter for the user to specify. Knowing the maximum flow is important for two reasons. First, you must ensure that the changeover has sufficient capacity to feed the process. Second, you must ensure that the gas sources are sufficient to meet the operating parameters of the application.
6. Determine the desired gas source change out frequency.
To determine the gas source frequency change out time, consider the flow rate, the total hours of operation, and the time period to obtain full containers to replace the empty side of the changeover. Let us look at some examples.
Example 1: In a simple application where a changeover manifold with one high pressure helium cylinder on each side feeds a gas chromatograph, the flow rate is normally about 120 sscm 24/7. This flow rate is equal 1,693,440 scc/week or about 59 cubic feet per week.
Assuming that the customer is using a standard 288 cubic foot cylinder and draws it down to 200 psig at changeover, one cylinder will typically last about 4.5 weeks. This is certainly sufficient time to replace the empty cylinder with a full one to recharge the system to switch back the other way. Here, the change out frequency is one month.
Example 2: In a more complicated scenario, let us assume that the changeover has one cryogenic container of nitrogen on one side and we need to know how many high pressure cylinders are needed on the other side.
The user’s gas usage is 50 SCFH – well below the maximum sustainable flow of 325 SCFH. The process operates eight hours per day, five days per week. The typical useable capacity of a cryogenic container is 5,000 cubic feet.
Under these conditions the changeover will switch to the high pressure reserve side in 12.5 days. If the supplier can provide a new liquid container in three days, the high pressure side will require five 300 cubic foot cylinders of nitrogen to ensure a continuous supply of gas for the process. If the replacement delivery is two days the high pressure side may be reduced to three cylinders.
Example 3: For a 24/7 system, the reserve side must have sufficient capacity to last for the longest conceivable duration required for replenishment. For example, if your “in service” side runs out at 6:00 p.m. on a Friday after everyone has gone home the “reserve” side must have enough gas to last until Monday morning – if there are enough cylinders in your storeroom as backup.
Figure 3. New changeover manifolds like the Ultra-Logic are operated via touch-screen, have data logging capability to help users comply with 21CFR11, can be fitted with automatic purging, and can be fully controlled over the internet.
Otherwise, the user now needs enough cylinders on line until whatever day his supplier can deliver fresh cylinders. However, don’t forget that some weekends are holidays like Thanksgiving. On these occasions companies can close for four days and many workers add-on vacation days. Your reserve supply calculation must also accommodate this eventuality.
Summary
Whatever your requirements, be sure to consider all of the elements in choosing your changeover manifold. Proper planning up front will provide benefits and smooth operations for many years.
When planning your next system take a look at the current market. Besides the old standbys, there are exciting new units like the Auto-Logic II pictured earlier and the Ultra-LogicTM pictured in Figure 3. The Ultra-Logic has data logging capability to help you comply with 21CFR11, can be fitted with automatic purging, and can be monitored and controlled from anywhere via the internet. SGR
Frank Scornavacca
Frank Scornavacca is president of SGD, Inc., a single-source, full-line wholesale specialty gas distributor with a unique support program. He can be reached at:
frank@sgd.com
