Control Valve-Flashing Service-Velocity at the Valve exit

For a control valve in flashing service, is there a guideline on how much velocity at the valve exit should be? Since erosion caused by flashing varies as sixth power of velocity, I think that valves sized for flashing service application should be sized based on restricting this velocity to some standard value. 

Usually, it is considered sufficient to select a hardened trim material based on the process fluid application. There are a few vendors who usually provide calculations for the velocity at the valve exit and they size the valve based on that velocity to not exceed a value specified in their documentation. For flashing applications, they usually assume a homogenous liquid vapour mixture and do calculations. 

Apart from control valve selection criteria which should primarily focussed on not just the valve capacity but also valve dynamic performance specifications (resolution, dead band and T86 response time) and valve gain, what can be done to select a valve for a flashing service application which is sized according to restricting velocity at the valve exit to some recommended standard value, increase the life of selected hardened trim material and still meet the aforementioned valve selection criteria? 

2 Replies

  • Great questions Saurabh,

    I reached out to our Fisher Valve severe service team and they provided the following response. I'd also recommend viewing some of our content on flashing:

    There are three major barriers that make velocity calculations for flashing liquids very difficult. These are:
    1. Availability of Complete Thermodynamic Data for the Flowing Media
    In order to attack this calculation, the complete thermodynamic data for the flowing media must be known. While this may be present for single component fluids, i.e. water and the steam tables, multi-component fluids such as hydrocarbons make it nearly impossible to know what is occurring in to the flowing media to each of the individual components for every pressure and temperature that might be encountered. There are simulation programs that can closely predict the data for each individual component, such as SimSci or Hysim, but this is the first obstacle to be overcome.
    2. Distribution of Vapor in Flashing Liquid
    Even if the thermodynamic data is completely known, the distribution of the vapor in this flashing liquid causes another problem. There are a multiple of different flow regimes that describe how the vapor is distributed in a particular application. Slug, wall, channel, annular, and stratified flows are some of these examples. In order to make a velocity calculation manageable, homogeneous flow is normally utilized. This assumes that there is a perfect mixture of liquid and vapor throughout the entire flow area. In the light of the other flow regimes, a calculation using this assumption would not provide accurate results because there is no way to know which flow regime would actually occur.
    3. No Industry Accepted Implementation Guidelines
    Finally, if a velocity calculation is completed there are no industry accepted guidelines as how to apply this information in regards to control valve trim design and material selection. Any direction that is available does not differ from that of a flashing application using the general service conditions of pressure, temperature and flow rate.

    It is due to the constraints listed above as to why the request is so difficult. If there are any questions or comments regarding the information above, please contact your nearest Emerson sales partner.

    What is Flashing webpage

    Fisher Catalog 12: Section 2 for Valve Sizing

    Control Valve Handbook: Chapter 5 Control Valve Sizing

  • In reply to mark.nymeyer:

    Thanks Mark for taking your precious time in answering my question.

    I had been in touch with one of Emerson's sales partner and received the same set of three reasons for not been able to carry out calculations of velocity at the valve exit for flashing applications.

    But for applications where reason 1) and 2) in your reply are not of any concern for the application in question, what remains is reason 3) which informs that there are no industry standard which address it.

    Hence, I wrote this post to initiate discussion on this forum on what can be done to address this gap and what reasonable approach can be arrived at.