The main crude oil pipeline is DN200; why is the flow meter and control valve assembly reduced in diameter to DN100, while the branch pipeline is DN150?

I. The diameter of the process pipeline is determined based on the properties of the crude oil and its flow rate. For control valves, the valve size is first determined by calculating the flow capacity value Kv or Cv under the given process conditions; the valve is selected based on a value that is about 1.5 times this figure. Afterwards, the opening degree of the valve is calculated so that its normal operating range lies between 60% and 80%. II. If the KV value of a single-seat control valve with DN100 size is 200, this means that when there is a pressure of 1 kilogram across the valve, the DN100 control valve can handle a flow rate of 200 m3/h. At this time, the process flow rate required is 200 m3/h, with a flow velocity of 2 m/s; therefore, the process pipes should be sized at DN200. For your control valve, a DN100 size will be sufficient. The valve is smaller than the pipe, so a diameter reduction is required. III. As for the diameter of the auxiliary line, it is likely that the design deemed 150 to be sufficient for normal production requirements; moreover, in general process design, the diameter is often chosen to be slightly larger, possibly as a cost-saving measure.

For cost reasons, flow meters with high precision are generally installed for feeding and discharging in such devices. However, these flow meters are expensive, and the price difference increases significantly as the size of the meter increases; this is why, as the original poster observed, the flow meter on the main pipeline is smaller than the bypass valve.

What was said on the 2nd floor is absolutely right; I’ve learned it! :) This situation is very common in practice. The control valve assembly consists of a control valve, a shut-off valve, and a bypass valve. The shut-off valve is used to close the pipeline during maintenance of the control valve, while the control valve itself is used to regulate flow when it is not in use due to maintenance. If the process requirements do not specify otherwise, to save costs, the shut-off valves and bypass valves in the control valve assembly can be smaller than those in the process piping.

I agree with what was said on the first floor; it’s very professional. But I think there’s another reason as well: a pressure difference needs to be created in order to achieve accurate conversion