Problems arise all the time in the field when operators have unique situations, and OTA is always here to help. One recent problem came from a client that has a VRU and VRT process setup and is working “like a champ,” the client said. However, they also combine the heater-treater (HT) gas with the VRT gas upstream of the VRU. “We are constantly fighting issues,” exclaimed the client.
There are a couple of problems that need to be addressed for this setup to be effective, and it is a very effective means of capturing flash gas. Firstly, HT’s rely on dump valves to exit product when a dump cycle occurs. When the HT dumps, it is dramatic. At the same time, product is flowing into the HT and flashing lighter ends (e.g. methane) and that gas stream is piped downstream of the VRT to commingle with the heavier flash gas from the VRT. There is no room for expansion in the downstream piping and this is a slug load that can overwhelm the VRU if the controls are not setup to handle this alternative operation. Lastly, HTs will occasionally overflow product into the gas stream, flooding the VRU with the likelihood of causing extensive damage.
The short term solution is to pipe the heater-treater gas into the product line flowing into the VRT. This will soften the spikes and allow more expansion into the gas cap of the VRT. In addition to piping the gas upstream of the VRT, restrict the flow (with an orifice or other means) to diminish gas spikes. With the HT gas output piped into the product stream flowing to the VRT, when and if the HT overflows it will rejoin its constituents in the VRT, instead of flowing into the VRU inlet and flooding it. Problem solved!
The long term solution is to replace the standard top-to-bottom VRT with OTA’s Max bottom-to-top VRT, which simulates storage tank flash, working, and breathing losses. The overall structure can be reduced because of its greater efficiency, resulting in lower material costs. HT gas can be directly injected into the gas cap of the Max VRT without having to regulate the flow. In addition, the product flow from bottom-to-top greatly increases flash rate because the product gradually is reduced in head pressure until it reaches near atmospheric. This will drastically soften and decrease product and gas spikes tremendously.
As always, call your local sales representative with any questions.