Applications at ammonia plants can be corrosive and damaging to your equipment. The pressure instruments you use must be able to stand up to the harsh environment of ammonia applications, or you may face equipment damage, operator injury or downtime for your process.
So, what pressure gauges and accessories are the best choices for use in ammonia plant applications?
This article will discuss some of the factors you should consider when selecting pressure instrumentation for ammonia to help you find the best solution for your process.
Considerations when Selecting Pressure Instruments
When you’re selecting the best instruments for pressure measurements in ammonia applications, make sure to keep these factors in mind:
Material
The materials that make up the instruments must be able to withstand the effects of ammonia. Zinc, copper and brass are some materials that should not be brought into contact with ammonia that contains water because ammonia corrosion can occur.
Be cautious of materials that can be associated with ammonia processes. Hydrogen is combined with nitrogen to make ammonia through the Haber-Bosch process. Nitrogen is not compatible with glycerin, so instruments containing glycerin should be avoided.
Viton is also not resilient to ammonia and should not be used. You can learn more about material compatibility with our material selector/corrosion guide on our website.
Instrumentation components should be made from 316 Stainless steel, carbon steel or silicone. Solid-front gauges are also important to use in these applications because they can help keep operators safe if a gauge bursts while they’re reading the dial.
Temperature Effects
Liquid ammonia averages minus 32 °F so make sure your pressure instrument is rated for that temperature. If it isn’t, you must use capillary line between the process and the instrument to offset the low-temperature conditions.
Steam is often utilized as part of the ammonia production process. Heat dissipation components such as coil or pigtail siphons should be installed between the instrument and process to protect your instrumentation from the effects of steam. Keep in mind that, as with any steam application, siphons must be primed during installation (water added to the lower half of the coil) to create a barrier between the direct steam and the wetted parts of an instrument.
Pulsation, Vibration and Pressure Spike Effects
For applications where pulsation and vibration are present, a dry pressure gauge can be difficult to read and have a limited life span. The effects of vibration can be mitigated by using a liquid-filled gauge or by removing the gauge from the source of vibration.
Liquid fill, typically glycerin or silicone oil (though, as mentioned earlier, glycerin should be avoided in ammonia applications), will lubricate the movement, dampen pointer motion, and change the resonant frequency of the gauge resulting in increased service life.
In extreme conditions, a gauge can be mounted remotely and connected to the process using a flexible capillary line. This allows the gauge to be mounted on a column, bulkhead, or panel that is isolated from the source of vibration. A capillary will also function as a flow restrictor, reducing the severity of pulsation.
Ashcroft’s PLUS!™ Performance is an award-winning dry case pressure gauge that dampens pulsation and vibration and improves gauge readability. This proprietary Engineered Dampening Media (EDM) of a gauge with the option is injected into a canister that surrounds the gauge movement pinion shaft, movement plate, and backplate (see figure 1 below).
Figure 1: Diagram of Ashcroft’s PLUS!™ Performance
This movement technology utilizes a thixotropic liquid that adheres to the pinion. As gauge pressure increases or decreases, movement of the gauge pinion creates shear load, which causes EDM molecules to scatter. The EDM material gets thinner at the layer being sheared. With applications involving vibration, this allows dampening of the gauge pointer. This action allows for a steady, flutter-free pointer.
Overpressure Effects
Some pressure gauges may experience overpressure in an ammonia application. Overpressure may deform the bourdon tube resulting in a calibration shift and will increase the operating stress on the bourdon tube. High operating stress increases the risk of metal fatigue and can result in bourdon tube failure and loss of process containment.
Typically, a pressure gauge should be selected so that the operating pressure is about 50% of the full-scale pressure. ASME B40.100 recommends that the normal operating pressures for a gauge should be confined to 25-75% of the full scale.
Ashcroft Recommended Products for Ammonia Plants
Most Ashcroft products with 304/316 Stainless steel wetted components can be used in ammonia applications. Take into consideration pressure and temperature ratings for each instrument selected regarding the process conditions as you would with any other application.
Ashcroft’s 1209 pressure gauge and 1279 pressure gauge are both good choices for harsh applications in ammonia plants.
For heat dissipation in high temperatures, Ashcroft’s 2198 MicroTube™ siphon protects pressure instruments from the effects of elevated temperatures. The reduced internal volume allows for superior temperature dissipation and can also be used in conjunction with diaphragm seals.
We don’t like to pressure you, but we have more information.
Now that you know the factors to consider when choosing the right pressure instruments that can stand up to the hazards of ammonia, you can research the solution that’s best for your ammonia application.
Keeping these factors in mind can help ensure that you avoid problems and keep your process running with reliable and accurate pressure measurements.
If you want to learn more, you can read some of our other articles:
- Fill Fluid Options for Pressure Instruments
- Heat Dissipation: Pressure Gauge Accessories for Non-Steam Applications
- How Does Temperature Affect Pressure Gauge Performance?
- Why Did My Pressure Equipment Fail? 6 Instrument Killers
Feel free to contact us today to talk to one of our industry experts and get your questions answered.
And if you’d like to learn more about keeping your equipment running, download our eBook:
