When it comes to pressure vessels, flare knockout drums, and wellsite separators, we often find that the inlet and outlet nozzles are undersized. They are not designed as per the diameter and length of the equipment. This limits the use of the device because the inlet, outlet, liquid dump, PSV, instrument, and drain line nozzles restrict the flow through the vessel. Thus, the process designer and the pressure vessel designer must work together to ensure that the nozzle size is right.
Aspects to Consider for Pressure Vessel Nozzle Design
- Upsize each nozzle during the initial construction of the pressure vessel to provide flexibility should the need arise in the future.
- Maximize the size of the nozzle. Then, swage down the piping just before the vent instead of matching its dimension to the diameter of the connecting pipe.
Types of Nozzles for Flare Knockout Drums & Pressure Vessels
Inlet and Outlet Nozzles
It helps to design and size the inlet and outlet nozzles to match the connecting pipe system. The system is sized to meet the velocity and pressure drop requirements based on a given flow of fluids. Flare KO drums and other equipment operating at low pressures have nozzles with a larger diameter as compared to higher operating vessels for a given flow rate.
Calculating the Maximum Flow for Inlet & Outlet Nozzles
- Standard Pipe Diameter Sizing
It helps to take a standard industry velocity value of 0.5 – 5.0 m/s for liquids and 10 to 20 m/s for gases. Also, 1.0 to 2.0 m/s is a good starting point for two-phase flow.
- Actual Mechanical Geometry of the Nozzles
Internal projection configuration impacts the selection of the discharge coefficient. It also helps to know that inlet deflectors and diverters adjacent to demister pads and the nozzles upstream of outlet nozzles will have a higher impact on restricting the flow. They must be calculated separately and considered when determining the overall throughput of a pressure vessel.
PSV Nozzle Connection
As per ASME Section VIII, the nozzle and pipe diameter cannot be smaller than the inlet connection of the required PSV capacity. Also, the PSV nozzle connection must be sized to relieve the flow of fluids if the operating pressure reaches a level higher than the maximum allowable working pressure (MAWP) that the vessel was designed for.
Liquid Outlet and Drain Nozzles
Since pressure vessels are primarily used to separate liquids from the gases, the liquid outlet nozzle sizes are as crucial as the inlet and outlet nozzles. It helps to know that the size of the drain nozzle connection must be according to its service. A typical velocity of 3.0 m/s and an appropriate pressure drop can be used if the liquid is clean and contains minimal solids. Be sure to size the lines connecting to a pump suction accordingly.
Problems arise when you repurpose the pressure vessel from relatively clean service to a dirtier service such as an inlet separator to a plant. A Nominal Pipe Size of 2 inches is the preferable choice for inlet filter separator drain connections, and it should not be smaller than 1 ½ inch NPS.
Instrument Connections for Flare KO Drums & Pressure Vessels
Pressure vessels use a variety of instruments to control pressure, level, or flow in and out of the vessel. Level sensing instruments are of concern, especially if they are required to protrude inside the device. The inside diameter of a nozzle must be wide enough for the level displacer or float to fit through it. The same is applicable for thermal wells used with temperature gauges.
Fortunately, pressure vessels with undersized nozzles are easy to correct. Aspire’s pressure vessel designers and fabricators have the experience to economically modify any equipment so you can use it in a different service. We also have an in-depth knowledge of flare knockout drum sizing and design to help you.
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