PRESSURE REGULATORS EXPLAINED TESCOM TECHNICAL TRAINING (click on the title to view the complete technical training article from Emerson Process - Tescom)
Pressure Reducing Regulator / Valve (PRV)
The function of a pressure reducing regulator is to precisely reduce a high upstream pressure of a gas or liquid (from a cylinder, compressor, pump, etc) to a lower, stable pressure for the user’s application. Furthermore, the regulator will attempt to maintain and control the outlet pressure within limits as other conditions vary but the regulator will not control flow, only the delivery pressure. A regulator is also not to be used as a shut-off device as there is always a small amount of leakage across the seat. A shut-off valve must be used downstream of the regulator if isolation is required.
Back Pressure Regulator / Valve (BPR)
The function of a back pressure regulator is to limit and precisely control the upstream pressure of a gas or liquid (from a tank, pump, etc) and is much more accurate than a relief valve. Most direct spring operated safety relief valves have a high reseating pressure which is inconsistent and unreliable. This the primary difference between a safety relief valve and a back pressure regulator. A safety relief valve is designed to protect downstream personnel and equipment should over-pressurization take place. As such, when it's set pressure is overcome, it will blow wide open immediately and exhaust all of the pressure. It needs to be able to handle the full flow of the system in order to rapidly exhaust to protect downstream apparatus. A back pressure regulator is not a safety device it is designed for precision upstream pressure control. When the
regulator set-point is overcome, it will "crack" open (not blow wide open) and try to exhaust just the excess pressure above the set-point. When it cracks open, it uses its sensing element (relief valve's do not have sensing elements) to try and reseat very close to its set pressure. Most Tescom back pressure regulators have "crack to reseat" pressures less than ± 2% of the set-point (relief valves are typically ± 10%).
Certain speciality gases used in the semiconductor industry such as Hydrogen Chloride (HCl), Nitrous Oxide (N2O) or Carbon Dioxide (CO2) have a high Joule-Thomson coefficient. This results in a significant cooling effect when these gases expand in the gas distribution system on their way to the respective process. Especially the use of HCl rises the risk that remaining residual moisture is condensating forming hydrochloric acid, causing corrosion to the whole gas supply system but especially to the pressure regulator where the cooling effect is highest. Commonly used heat tracing cables have low heat transfer and only heat fraction reaches the inside of the regulator body.
Why is my regulator freezing? (Joule-Thompson Effect) (Application story by Louis J. Arcuri)
Have you ever seen a regulator that was encased in a ball of ice on a hot summer day?
It is strange to think that a regulator would be buried under a frozen mass of water when the ambient temperature is high and the rest of the piping is not frozen! Chances are that you’re seeing the results of the Joule-Thompson Effect in action.
Just what is the Joule-Thompson (or J-T Effect) and why should it be important to you?
First described by the noted scientists James Joule and William Thompson in 1852, the J-T Effect, or J-T for short, is simply described as the cooling effect of a high pressure gas as it expands into a lower pressure area. We’ve all come to heavily depend on the practical benefits of the J-T Effect; think air conditioning, Yes, the J-T Effect is what gives us that splendid cool air in our home, car or office on a hot summer day. Refrigerant is compressed to high pressure which then flows through an orifice where it expands into the heat exchanger tubes of the air conditioner. A fan moves fresh air over the heat exchanger tubes which cool the air as it moves into the ducts and flows through your home, car or office. The gas warms as it absorbs the heat from the air and is re-circulated, compressed and expanded over and over again to maintain the cool temperatures we crave on those hot summer days. If you use a spray can of air freshener, deodorant or other product you will feel the can cool in your hands as you spray the product. You are feeling the effect of the gas expanding as you spray it, cooling the can.
The J-T Effect is responsible for that large ball of ice around the regulator we observed earlier. High pressure gas is fed to the regulator and expands as it flows past the main valve and through the seat into the P2 chamber then on to the process. The gas is flowing at supersonic speed as it expands out of the seat, cooling the body of the regulator as it flows. If the gas has a high enthalpy (stored energy) it will cool off a lot. If the gas has a low enthalpy then its cooling is minimal. The ice builds up on the regulator because the body of the regulator is cooler than the surrounding air; the cooling effect of the expanding gas is greater than the ability of the regulator to absorb heat from the surrounding air to offset the cooling. This allows the moisture in the air to condense on the body of the regulator in much the same way we see condensation form on the glass of a cold drink on a hot, humid summer day. If the regulator body is colder than 32°F, the condensation freezes on the regulato r body. Over time, the frozen condensate can grow into a substantial ball of ice, making the problem worse, as the ice prevents the regulator body from absorbing heat from the surrounding air. Certain specialty gases, such as Carbon Dioxide (CO2) and Hydrogen Choride (HCl) have a high enthalpy and are very susceptible to J-T. Ammonia is another gas with a high enthalpy and is often used in large, commercial and industrial refrigeration systems. The air conditioning system on the International Space Station employs ammonia as the refrigerant. Though the sight of an ice-covered regulator may be surprising, there is no real harm occurring to the regulator itself. Rather, there more likely may be a problem with the controllability of the downstream pressure and this is a problem to the customer. Controllability may be affected if the cooling of the gas is so great that the gas actually liquefies briefly in the regulator after it passes through the main valve. This liquid then vaporizes back to a gas as it moves through the warmer piping beyond the regulator. Vaporizing the liquid produces pressure surges that are uncontrollable resulting in unstable downstream pressures = not good.
There are several ways to deal with J-T and minimize or prevent the gas from liquefying. Often, we use a two-stage pressure reduction scheme to minimize the J-T Effect. By taking the pressure drop in two stages, the total cooling effect is split between the two regulators, each of which may be able to absorb enough heat from the atmosphere to prevent the gas from liquefying. For some gases, such as HCl, the enthalpy is so high that two stage-reduction alone will not prevent the liquefaction of the gas as its pressure drops. In this case, heat is applied to the piping before the first and second stage regulators, raising the gas temperature enough to prevent the gas from liquefying as it passes through the main valves of the regulators. For high flow HCl systems, heaters rated for several hundred watts may be required. Consider how hot a 100 watt light bulb gets, and you can better imagine the amount of heat required to prevent HCl from liquefying at high flows. For lower flow applications, simply separating the two regulators with a long length of tubing will usually allow the gas to recover enough temperature between stages to prevent liquefaction after the second stage reduction. Another approach is to use a vaporizing regulator such as the 44-4800, which employs heat exchanger tubes to warm the gas with integral electrical heaters or steam. The 44-4800 is an excellent choice for minimizing the J-T Effect in low flow applications. Sometimes, using regulators with larger bodies, such as the 44-3200 and 64-3200 series will help offset some of the J-T Effect as the larger mass of the body can absorb more heat from the surrounding atmosphere and requires more cooling from the gas to reach liquefaction temperatures.
Most gases exhibit a cooling effect when they expand; two notable exceptions are hydrogen and helium. These noble gases actually generate heat when they expand, though the heat generated is negligible. While it may seem the J-T Effect is undesirable, we have already seen a positive use for it in air conditioning. Another very important benefit of the J-T Effect is cryosurgery. Cryosurgery is used in removing warts and other unwanted skin conditions by flowing two gases at low flow, but under high pressure, through a surgical instrument that allows the gases to expand at the tip of the device. The expanding gases cool the skin and freeze it locally; the pressure of the gas then cuts through the skin to remove the offending condition. Cryosurgery is also used in a prostate cancer surgery procedure known as cryoablation. In this procedure, cryoprobes are inserted into the prostate gland. Argon and helium are circulated through the probes; the gases expand in the probes, producing the desired cooling effect. The cryoablation process freezes the tumor and kills the diseased tissue. Tescom makes a changeover panel, the NA-48, for cryosurgical gas applications. J-T is one of those practical applications of physics that we see every day, but don’t fully appreciate. Yet, without the beneficial effects JT, our lives would be much less comfortable!