In the oil and gas industry, there are instances where you would use natural gas to move control valves. So, why spend money on a compressor for producing instrument air and add electricity costs when you have ample natural gas available? There are issues, however, when using the gas for these applications. This is common if an electronic signal controls the valve.
Gas isn’t always easy.
The most important question you can ask yourself is: What is in the gas? Gas can be great, but it also may contain particulates, moisture and corrosive contaminants that will affect valve positioner performance.
Let’s start with particulates.
All valve positioners have small orifices. Particulates can cause these orifices to plug. Therefore, it is a common practice to put a filter regulator at the input supply. The filter should be approximately 5 microns. And, that solves that problem.
You can resolve this by using a coalescing filter. However, periodic maintenance (drainage) must be performed on the filter to remove the moisture. If you don’t perform this regularly, the filter can become saturated and moisture can get into the valve positioner and cause it to fail. Corrosives, too.
In the oil and gas market, H2S is commonly found in natural gas. This is a problem because unless the H2S can be removed before it gets to the instrument, the valve positioner will certainly fail. The best option here is to remove the corrosives from the gas. This is typically found in the later stages of the refining process.
But wait, there’s more.
There are electrical issues that need to be addressed. If the valve positioner is controlled by an I/P converter, then an explosion-proof housing is commonly used. If you are dealing with a digital valve positioner, then you are required to meet certain safety requirements. A circuit board needs to have hazardous approvals (i.e., FM/CSA/ATEX) and it has to have an intrinsically-safe circuit. That means that you need an approved intrinsic safety barrier in the circuit to limit the current, such that a spark will not occur if something goes wrong like a short-circuit.
So, what’s the best way to deal with it?
To avoid the electrical issues, you can use pneumatic control valves, but you will have to contend with excessive bleed. What this means is that you have increasing hazardous conditions (potentially from Class I, Div II to Class I, Div I area), and you also will experience EPA issues. The EPA has the Natural Gas Star program with partners who are key players in the industry. For a separate white paper on the emphasis on reducing the bleed of natural gas into the environment, please click here.
Or, you can capture the bleed and feed it into the process, but you can’t do that with all valve positioners. You can use electro-pneumatic valve positioners (I/P), but you will still have excessive bleed PLUS electrical requirements, as described in the paragraph above.
An attractive alternative would be a digital valve positioner with little or no bleed. Siemens offers this with the SIPART PS2. Not only does the PS2 offer the smallest bleed rate on the market, but the exhaust port is tapped so you can pipe away all valve exhaust gas. This works because of the SIPART PS2 piezo valve technology and tapped exhaust port.
Why is the PS2 the ideal alternative?
- It is one of the easiest valve positioners to configure and calibrate
- It consumes the least amount of gas
- It is one of the most accurate valve positioners on the market
- Exhaust gas can be piped away or captured with the integral tapped exhaust port
- It’s competitively priced It’s best for use with control valves