If there is no bypass for the valve assembly, do the upstream and downstream shut-off valves still serve a purpose?

There are cases where front and rear valves are needed, for example when the instrument air is interrupted and the control valve fails to function; moreover, control valves generally have poor sealing properties. In addition, in some intermittent operating conditions, it is necessary to stop the process without discharging material, and control valve failures are handled by shutting off the front and rear manual valves

The installation of manual valves before and after the control valve has no direct relation to the presence of a bypass. The fact that the control valve has no bypass indicates that it is critical here; if there is an abnormality in regulation, the system must be stopped, as using a bypass for adjustment is not possible. However, the materials inside the parking area are not necessarily emptied, so it is very necessary to have front and rear manual valves. It is very convenient to remove the control valve.

Someone raised the issue of leakage points; an additional bypass would create more leakage points

An additional bypass requires extra front and rear flanges, as well as three potential leakage points in the valve packing. The utilization rate of bypass valves is low, as they are generally kept closed. For processes that require continuous operation but involve pressures that are not particularly high, a bypass design is often chosen. This decision is the result of discussions between the owner and the design firm; for example, if the process is continuous but the pressure is high, a bypass design that involves two valves is necessary, which increases costs. In such cases, if both parties agree, it is possible to opt for shutting down the system in case of a valve failure. Since the failure rate of valves is currently very low, and with the owner’s approval, bypass systems are usually not used in batch operations. The purpose of this approach is simply to determine how to handle valve failures and what kind of solutions can be adopted

If it is a control valve, a bypass system is needed for maintenance purposes; but this is not necessary for a shut-off valve (which can be simply turned off or on). In the event of any operational issues (such as a failure of the power source), operations will be carried out in the safest manner possible (FC/FO)

The steam line double track can be used for purging and pipe heating

The steam bypass lines are different from the heating lines. The diameter of heating lines is generally between DN15 and DN50, with rarely any larger diameters. The design principles and concepts for heating lines also differ from those for bypass lines: bypass lines need to ensure a certain flow rate in the event of a failure, whereas heating lines do not require this. Moreover, using a bypass line as a heating line presents certain risks. When used for heating, the temperature of heating lines should not exceed the material’s limit for deformation, usually keeping it below 50°C per minute. Bypass lines generally have larger diameters, making them more difficult to control

Thank you! I didn’t know there was such a difference

However, I have seen a rather special crude oil pipeline: the main pipe is 20 inches in diameter and equipped with a control valve, while the bypass pipe is 6 inches in diameter and also has a control valve. There is also a 2-inch diameter bypass pipe with a control valve as well. The sequence of starting up the system is first to use the 2-inch pipe, then the 6-inch pipe, and finally the 20-inch pipe. The reason for this design choice is to reduce the back pressure during startup

Sure, there must be. If the control valve needs to be repaired, it’s still necessary to shut off the material flow using the valves before and after it, so that the control valve can be removed from the system