Ice Forming Along a Glycol Pipe Line

Glycol is a whole class of alcohols. You didn't say what type. Obviously, it's being used to cool an area away from the cooling unit and is below freezing temperature. The pipe should be insulated to avoid wasting energy.

Ethenol at diluted concentration of 25%

Freezing can occur not only when water in the gas stream mixes with temperatures below 32 degrees Fahrenheit, but also with the presence of hydrates well above the freezing mark. Freezing can occur in natural gas from gas wells and from produced gas from crude oil wells. It can occur at any point from production to delivery. Knowing the best ways to prevent freezing is critical. A free-flowing gas stream is a sure way to prevent headaches and save companies time and money. A small investment in prevention will pay off in a big way later in a company's bottom line.

The best way to prevent freezing is to anticipate where the problem is most likely to occur. Areas where there will be a drop in pressure or restriction in flow are likely spots for freezing. Temperatures drop about 7°F for every 100 psi pressure reduction, so even if the flow stream of gas is at a temperature above freezing, that temperature could drop below freezing with a reduction in pressure. For example, gas flowing through a pipeline at 70°F and 800 psi will not see any effects of freezing. However, if you cut the pressure in the pipeline to 100 psi, the temperature of the gas will drop nearly 50°F, plummeting it below freezing. If that gas has any water vapor or condensate present, it can lead to freezing problems. Knowing the quality of the gas and the pipeline design and operation are also important. Typically, the gas quality will improve from the time it is at production and gathering systems to the time it reaches Local Distribution Companies. High BTU gas is more likely to form hydrates or freeze. Hydrates form when water vapor combines with hydrocarbons to produce a compound that will condense and freeze at temperatures above the freezing point of water. They can form balls of ice that can severely damage or block the pipeline. Equipment, such as instruments, probes, and orifice plates, should be removed at the start-up of a new or cold well so that they are not damaged by the ice or hydrates. Once the flowing stream has been stabilized, they can be put back in place. Additionally, ice and hydrates can lead to inaccurate measurement. Ice forming on orifice plates can reduce the orifice diameter, leading to inaccurate results. Ice forming on controllers or regulators can cause them to malfunction or stop working completely. Even after the ice thaws, problems can remain with them. SOLUTIONS While freezing leads to many problems in a pipeline, there are several cost-efficient and effective solutions to prevent them.

The best way to prevent freezing is by dehydration, the removal of water from the gas stream. Simply stated, with no water, there is no freezing. Glycol One way to remove water is through glycol absorption. As gas passes through the glycol inside a contactor, the glycol meets up with a mixture of water vapor and hydrocarbons. The glycol absorbs water vapor entrained in the stream, allowing dry gas to pass through. The glycol is then treated by circulating it to a regenerator and distilling the water out of the glycol. The glycol is then put back in the contactor and the process is repeated. Glycol absorption systems are relatively inexpensive, but are not without some concerns. There can be some glycol carryover during surges. Changes in flow can affect the system's efficiency and there can be contamination by solid particulates. Sieve Another effective method of dehydration is through the use of molecular sieves in large towers. This solid absorption method is used when higher efficiencies of water removal are required. As gas passes through the large towers of the sieve, it is absorbed by the sieve. The sieve eventually becomes saturated and can be regenerated. As hot gas passes over the sieve to dry it and evaporate the water, the gas stream is directed to a second absorption tower. The sieve in the first tower is then cooled by gas before it is ready to be used again. When the sieve in the second tower is saturated, the gas is switched back to the first tower and the process is repeated. Very dry gas can be obtained using this method, although the process is more costly than the glycol absorption method. Methanol Another effective and inexpensive tool against freezing is injecting methanol into a pipeline. The methanol is put into the gas stream through a pump or drip and works as an anti-freeze by joining with the gas and water vapor to lower the freezing point of the vapor in the stream. Determining the right amount of methanol to use can be calculated using tables for specific applications.

Methanol injection is also sometimes used to prevent freezing in pneumatic controllers, as well as in preventing liquids from reaching small orifices and passages in these instruments. Addition filters are sometimes put in place to prevent methanol from carrying over into the instrumentation. Heat If gas never reaches freezing temperatures, it stands to reason that it cannot freeze. Applying heat will prevent freezing conditions. There are several approaches. Line heaters can be put in place. Heat can be applied directly to a control valve body where there are pressure and temperature drops. Direct heat can come through the use of a catalytic heater in an enclosed area. The heat will keep the temperature above freezing, but becomes less effective the farther away it gets from the heat source. Other possible solutions are the use of heat blankets and steam systems. For all the positive aspects of heating, there are several major concerns. Heat is an ignition point for gas. Safety precautions need to be put in place and followed. It can also be an expensive solution to freezing because of the equipment required. To maintain the heated environment, additional energy is also required, which means more expense. System Design Careful planning during the design stage for measurement and regulating systems can reduce the chances of freezing.

To avoid liquid accumulation, pipe configurations should be set up such that drainage slopes toward drain fittings in low spots. Prevent restrictions by using full opening ball valves and large diameter tubing. Liquids will be drawn toward leaks, so have a leak-free system with tubing that slopes back toward the pipeline. Any steps that reduce restrictions or prevent areas where liquids can collect will minimize the possibility of freezing. Drip Pots and Liquid Dumps In cases of severe liquid problems or when there is a slug of liquid in a gas supply used for instrumentation, drip pots and coalescers can be used to eliminate or reduce the amount of water. For more extreme cases, an automatic liquid dump might be a better solution because it can act as a drip pot collection vessel and will release the collected liquid at a lower pressure point. Instrument Filters Filter dryers provide a clean, dry supply of gas to controllers and other instrumentation that functions using instrument gas. These units function under high pressure and can eliminate both liquids and particulates. Their removable media cartridges can be filled with various media, from desiccant to charcoal, molecular sieve or material to remove dangerous H2S. They can provide protection down to 2-4 microns. For critical locations, where shutting down the pipeline is not an option, the filter dryers can be built into a dehydration system with offset regulators to provide uninterrupted service. If one side of the system shuts down due to desiccant that becomes saturated, causing the regulator to freeze, the other side kicks in so that there is no interruption in service. The media on the first side can then be replaced and the regulator thawed. The filter dryer can come with a tattletale desiccant that changes color when the desiccant becomes saturated, indicating it needs to be replaced. Another option is a system that provides regulation, filtration, heat, and a manifold all in one tower. It is designed for pneumatic controllers and process control instrumentation systems, but can be used for other applications that require a clean, dry gas or instrument supply system.

GA from me too .

It would seem the glycol is below freezing temperature for water(-0°C)....this would cause pipeline sweating below dewpoint, and then the condensation freezing over a short time, this then continuing, forming layer upon layer of ice...Insulation is required usually in this situation to prevent damage and conserve energy.....

must be a lack of carbon in the local air. either that or lowtemp

these are the famous ( and cool) fountains @ Belagio casino Vegas. when initially tested they all quit and formed a ball of ice on the valve, they began disassembling the "faulty" valves....they were fine you just explained their basic misunderstanding of why.

A wonderful place to visit for a short duration of time. (in most cases)

after 50 times or so its old hat

Thanks for these valubale information. makes me understand it more.

If I recall correctly, the cooling effect applies to gases, not to liquids. You may have gas bubbles in the glycol that respond to the pressure drop and cool the pipe, but I don't see how the cooling is a liquid phenomenon.

But SHOCKHISCAN: Doesn't this require that the glycol be in the gas phase? OP never stated the glycol was not liquid. If this glycol is part of a natural gas drying unit, it could be the natural gas is carrying over, and flashing off in a low pressure section of pipe downstream of a restricted control valve (perhaps, as you already noted, I think).

Otherwise, the drying unit is being run well below ambient temperature, resulting in very cold glycol exiting the gas drying unit.

let explain how it works in your fridge or AC. LIQUID is in a Hi pressure high temp state as it approaches the coil you want to get COLD ( usually called an evaporator) at this point the pipes carrying the liquid are more warm or hot. a metering device like a valve is the choke point which has caused the bottleneck so pressure could build in a system. once the liquid begins to pass the bottleneck it hits a lower pressure area. with the pressure suddenly coming off the liquid is now free to transform to a gaseous state. in order to do that it needs HEAT. as the liquid tries to boil to gas it pulls heat from all directions. cold is simply the absence of heat. as heat is drawn from the pipe or valve they get COLD if moisture is present it will form frost or ice. Shock dude had it right he just didn't go into the physics as far as I'm sure he could have. in my diagram you see an "expansion valve" is the throttle or bottleneck that holds back pressure as well as releases it to "feed" the evaporator the same "frost formation" of pressure release is seen in welding tanks or an other vessel that bleeds off pressure.

but with "glycol-ethanol" as the "refrigerant"? Was totally not aware that would produce much chilling at all.

I wouldn't call it a refrigerant at all but both have boiling points and can change from a liquid to a gaseous state so with pressure their boiling point changes..not to mention impurities...water/ air .

My Friends, the ethonal concentration does do pass through a ammonia refrigeration cooling Plate heat exchanger. therefore its chilled fluid

Then no wonder the pipe might have ice or frost on it. Once upon a time, it was glycol; now it's ethanol. More accurate detail of this whole process is needed, such as design temperatures on both sides of the heat exchanger.

Agreed about glycol or ethanol, you beat me to it. But neither would give significant cooling on throttling, as others have said.

A refrigerant is chosen so that it is liquid upstream of the expansion valve but evaporates and cools going through the valve (Fredski's diagram in #12 is not quite right IMHO, but it gives the general idea). Ammonia, BP -33° at atmospheric pressure, was I believe one of the first to be used, but superseded due to toxicity.

Ammonia is still numero uno choice in many industrial chilling operations. I think it is used in some cases with stack vapor recovery.

In fact ammonia can be used in a cycle that is essentially a Rankine cycle, in an ammonia- water binary system where heat exchangers produce a change in composition that allows extra recovery of energy

Sorry about the "giant picture", but at least it illustrates the "sliding condensation temperature" on the T-S diagram.

OK thanks. I'm sure you're right, it's not my field, but I'm fairly sure about the basic principles.

You mean we spent x hours at a rate far exceeding $50/hr in most cases, only to be told late in the game that the ethanol-glycol system was already chilled? Are you serious you did not know why the pipe got cold? What planet are you from?

James, the refrigeration that cools the alcohol is a ammonia refrigeration plant. let that be clarified.

it means simply that glycol at that part of the pipeline undergoes depressurization or expansion.

Is the pipeline originally not insulated?or insulated?