# Calculating velocity of steam through orifice

1. Jul 12, 2016

### tobitronics

Hi all,

First time poster here and I'm stuck with what for me seems to be a simple problem. The situation is this:

I have a 100mm diameter pipe connected to a steam kettle, which produces steam at a constant 0.3barG. This pipe is split up into 4 50mm pipes, each pipe has a valve at the end that can be opened and closed and an orifice downstream with a diameter of 37mm. The pressure after the valve is atmospheric (0 barG). The manufacturer has given me a figure of consumption of steam per hour, although I do not know under which conditions this number is calculated. The steam passing through has a temperature of 108C, which corresponds to the pressure.

I want to calculate the velocity of the steam when its passing through the orifice (and the velocity when no orifice is applied). I know it has something to do with bernoulli's law and I tried using the calculators at TLV.com, however I seem to get contradictory answers.

F.e. when I calculate the flow rate through the orifice and use that number to calculate the velocity through the pipe (flow rate is constant throughout the pipe?) I get results which would indicate that not using an orifice increases the velocity, which (of course?) is not true.

Can anybody point me in the right direction?

2. Jul 17, 2016

### Greg Bernhardt

Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

3. Jul 18, 2016

### JChemP

The pressure downstream of your valve will not be atmospheric. As the RO is there to restrict the flow the pressure upstream will be 0.3 barg.

The flow through the orifice can be calculated using nozzle flow equations (See CRANE technical paper)

The length of line downstream of your RO also would affect the result. How much pipe is downstream of the RO? If you have a long length of line you should do a line sizing calculation applying a K factor for your RO.

The RO is there to cause a pressure drop, so the velocity downstream will certainly be greater. The max velocity through the RO will be at the vena contracta.

The velocity downstream of the RO will also not be constant. It will change depending on the pressure. The pressure at the discharge of the piping will be either atmospheric if the flow is not choked, or greater than atmospheric if the flow is choking. Either way the pressure will be greatest at the RO discharge and reduce along the pipe i.e the velocity will not be linear. It will increase along the discharge line.

Check out API Part 1/2 and Crane.

What was your method for calculating the velocity?