Steam Drum PSV Inlet losses

Your pressure safety device should be properly sized to facilitate proper relief flow...

https://www.scribd.com/document/349284868/PSV-Sizing-Calculation-Spreadsheet

You must first know the volume, temperature and pressure that is required for this installation...Then you size the piping and valve flow characteristics to requirement...

" Here are six factors that must be considered when selecting and sizing safety or pressure relief valves.

1. CONNECTION SIZE AND TYPE

The valve size must correspond to the size of the inlet and discharge piping. The National Board specifies that the both the inlet piping and the discharge piping connected to the valve must be at least as large as the inlet/discharge opening on the valve itself.

The connection types are also important. For example, is the connection male or female? Flanged? All of these factors help determine which valve to use.

2. SET PRESSURE (PSIG)

The set pressure, which is measured in pounds per square inch (PSIG), is the pressure at which a safety or pressure relief valve opens.

The set pressure of the valve must not exceed the maximum allowable working pressure (MAWP) of the boiler or other vessel. What this means is that the valve must open at or below the MAWP of the equipment. In turn, the MAWP of the equipment should be at least 10% greater than the highest expected operating pressure under normal circumstances.

3. TEMPERATURE

Temperature affects the volume and viscosity of the gas or liquid flowing through the system. Temperature also helps determine the ideal material of construction for the valve. For example, steel valves can handle higher operating temperatures than valves made of either bronze or iron. Both the operating and the relieving temperature must be taken into account.

4. BACK PRESSURE

Back pressure, which may be constant or variable, is pressure on the outlet side of the pressure relief valve as a result of the pressure in the discharge system. It can affect the set pressure of the upstream valve and cause it to pop open repeatedly, which can damage the valve.

For installations with variable back pressure, valves should be selected so that the back pressure doesn’t exceed 10% of the valve set pressure. For installations with high levels of constant back pressure, a bellows-sealed valve or pilot-operated valve may be required.

5. SERVICE

Different types of service (steam, air, gas, etc.) require different valves. In addition, the valve material of construction needs to be appropriate for the service. For example, valves made of stainless steel are preferable for corrosive media.

6. REQUIRED CAPACITY

Safety valves and relief valves must be able to relieve pressure at a certain capacity. The required capacity is determined by several factors including the geometry of the valve, the temperature of the media, and the relief discharge area.

The required capacity is set by the applicable code. It is usually expressed in:

• LBS/HR – pounds per hour (steam flow),
• SCFM – standard cubic feet per minute (air or gas flow), or
• GPM – gallons per minute (liquid flow).

These are just the basic factors that must be considered when selecting and sizing safety valves and relief valves. You must also consider the physical dimensions of the equipment and the plant, as well as other factors related to the environment in which the valve will operate."

https://alliedvalveinc.com/the-valve-expert/select-size-safety-valves-pressure-relief-valves-tgc/

https://www.spiraxsarco.com/learn-about-steam/safety-valves/safety-valve-sizing

One can always transfer the burden of sizing and selecting the <...PSV...> to the local friendly valve distributor in pursuit for a quote for a suitable valve. However, that organisation will require a great deal more detail of the application than has been revealed so far in this forum; process conditions such as temperature, pressure and flowrate are among them.

If in doubt, consult a suitably qualified Process Engineer.

We are all having to make a large number of assumptions as you have not given us all the information needed so herewith my best guess

I think from your description of the piping arrangement you would need to include

Exit loss from the steam drum to the piping

Pressure drop along the larger diameter piping

Pressure drop through the reducer - normally the relief valve is mounted on the reducer so there is no length of lower diam pipe but if that is not the case in your installation this will be needed but must be short

I do not believe that there is any need to include exit loss from the reducer as in service the gas will maintain the velocity to the inner workings of the valve

The 3% rule is to the UPSTREAM side of the valve ie the set pressure of the valve so any pressure drop through the valve and to the nominal atmospheric pressure is not relevant

So if the RV is set at 70barg the pressure drop in the inlet piping (including exit loss from steam drum) cannot exceed 2.1barbetween the drum and RV inlet flange