Why are there two chokes shown in figure 3—2. Certainly, gas flow could be controlled with a single choke. One reason is that the erosion of the choke is reduced by limiting the pressure drop through a single restriction. Note the pressure profile between the wellhead and high pressure separator shown in figure 3-1. The other rationale for utilizing two chokes on high pressure wells is to prevent hydrate formation.
A following chapter, presents the causes and cures of pipeline freeze-ups. Suffice it to say here that excessive throttling across a choke will form a water-hydrocarbon solid inside the choke. To prevent this, the flowing gas is partially reheated as follows:
• Gas flows to the surface at 3500 psig and 130° F.
• The gas pressure is reduced to 2400 psig across the fixed wellhead choke. As a consequence of this pressure drop, the gas cools to 80° F.
• The gas stream is reheated to 140° F in the first loop through the heater.
• The adjustable choke—which is an integral part of the heater, throttles the gas pressure down to 1100 psig. This pressure reduction again cools the gas to 80° F.
It is clear from the above data that attempting to break a 3500 psig wellhead pressure down to the 1100 psig separator pressure across a single choke would cause the choke to freeze up (1100 psig natural gas may form hydrates at temperatures below 70°F) and the gas flow to cease.
To illustrate this idea, let’s assume that a heater’s adjustable choke is freezing up. The heater is operating as hot as possible. To overcome this problem, install a smaller fixed choke in the wellhead. This will permit operating with the heater’s adjustable choke in a more open position and hence reduce the temperature drop across the adjustable choke.
Inadequate heater capacity can be caused by a low water level. Exposing heat transfer tubes to air also accelerates exterior corrosion of these tubes.
Heating natural gas from 80° F to 140° F as described in the above example consumes about 0.2% of the well’s gas flow. While this is not much of a loss, always keep in mind that hotter gas reduces compression capacity and creates dehydration problems at downstream facilities. Hence, heaters must be shut down when the danger of hydrate formation expires.