In almost every plant, we need to install control valve, shutdown valve, and/or blow down valve. These valves can be operated by power supplies from pneumatic system, hydraulic system, or electrical system. In this post, I want to share you pneumatic power supply for instrumentation and general guidelines for selection.
[Read also: Advantages and Disadvantage of Pneumatic Power Supply and Electrical Power Supply for Instrumentation]
The pneumatic power supply is more commonly known as the instrument air system. However, in fact, other type of gas can be used as pneumatic power supply, such as nitrogen or natural gas.
There are several considerations if we choose instrument air system:
1 – Adequate Capacity
The minimum capacity of instrument air system should be the summation of individual of each air-consuming instrument (control valves, shutdown valves, blow down valves), plus contingency to accommodate leaks, purges, etc.
Instrument vendor usually have the data of instrument air consumption for the specified instrument. If the data is not available, estimated consumption of 0.5 cubic foot per minute for each instrument is usually adequate.
[Read also: Step-by-step Sizing of Air Compressor and Instrument Air Dryer Capacity]
[Read also: Spreadsheet of Air Compressor and Instrument Air Capacity]
The buffer tank that used to store air should have sufficient capacity to maintain the consumption rate for about five minutes or duration which is considered adequate to perform emergency shut-down of the plant. In my experience, the selected duration can be as high as 30 minutes or one hour.
2 – Filtering and Regulation
Instrument air should be free from all contaminants, such as water, oil, corrosive and hazardous gases. If possible, non-lubcricated compressors should be used. If it is not, oil removal separator is required. The presence of oil may cause instrument contamination and possibly create combustion mixture.
Before the instrument air is being distributed to the system, it is usually filtered to remove particulate carryover from the dehydrators or air dryer units.
3 – Proper Distribution
The distribution of instrument air system should be free of any pockets where liquid could accumulate. If this is not possible, drain valves should be installed. In many cases, instrument air filter and regulator should be provided at each air-consuming device to reduce air pressure to the supply pressure recommended by the instrument manufacturer. Filter is used to protect the instrument from contaminants. All supply lines should be connect to the top of the air manifold or header.
In some cases, non-air systems, such as natural gas has been used instead of instrument air system. It is usually used in remote area where instrument air was not available. This practice should be avoided if possible due to safety and pollution problems. We also need additional filtering and gas clean up to protect the instruments.
Some small plants have used bottled nitrogen for instrument gas. This is quite acceptable, but non-bleed type instruments should be used to keep consumption to a minimum.
My Experience Related to Pneumatic Power Supplies Selection
In one project, I have studied intensively how to select pneumatic power supplies for instrumentation. At first, I selected instrument air system due to its common application. The consumption is relatively low. To provide instrument air system, I needed to procure air compressors, instrument air dryers, buffer tanks, and piping system, which is quite costly. On the hand, the power consumption also increased due to compressors. The problem is, the plant was relatively simple and small.
From that experience I learnt that if we selected instrument air system, we need:
- Relatively large investment cost to procure air compressors and air dryers
- Higher power consumption for compressors (means higher operational cost)
Then, I studied what if I used natural gas for power supplies. It does not need quite many facilities compared to air system, however, based on previous experience, the price of natural gas actuated valves are double compared to air system. And also, it still needs filtering and cleaning for natural gas before entering the instruments. For gas facilities which produce relatively low gas, utilization of natural gas for power supply can lower production rate.
I also studied utilization of bottled nitrogen for power supply. In my case, bottled nitrogen is better selection than natural gas or instrument air system, especially when air consumption for the instrument devices is relatively low. In term of operational cost, at certain natural gas price, both nitrogen and natural gas are comparable. However, if the price of natural gas is higher than certain point, bottled nitrogen is cheaper.
The cost of bottled nitrogen is about US$ 0.28 per normal cubic feet.
Conclusion
- If we select pneumatic air systems for instrument power supply, we need to consider that its capacity is adequate, its quality meets instrument-device manufacturers recommendation, and its distribution is correct.
- Air system is not the only option for pneumatic power supply. There also also natural gas power supply and bottled nitrogen power supply.
- Selection of pneumatic power supply should be done case by case. This is because the application in one case may be not applicable for other case, in terms of technical, capital cost, and operational cost.
References:
[1] GPSA Section 4 : Instrumentation
[2] ISA-RP60.9-1981 Recommended Practice Piping Guide for Control Centers
[3] Cost of Nitrogen (https://www.mvsengg.com/blog/cost-of-nitrogen/)