Surface Equipment – Heater Operation
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.
Categories: Wellhead Surface Equipment | Tags: Heater Operation, Surface Equipment | Leave a comment