For those poor souls with no Chemical Engineering background

As you can probably see by the title, this blog is dedicated to process modelling. In particular to Process Modelling with HYSYS. If that doesn't ring a bell to you, you better not continue reading or maybe have a look at http://kwizim.blogspot.com. Although that blog is only updated very rarely, you may find some info that allows you to know me better from a non-professional perspective. For the day to day gossip, try sites like Twitter, Facebook and Dopplr. Or just Google Kwizim.

If you would like some more info on heat exchangers, you could have a look at: http://jcazenave.blogspot.com

Tuesday, 11 February 2014

Modelling Valves in HYSYS Steady State and Dynamics

In HYSYS Steady State, for a long time the valve would only work with a specified pressure drop. However, as of version V7.3, there is a check box on the Design / Parameters page. that will make the valve calculate the pressure drop as a function of valve opening and valve characteristics. You should be aware that for an imposed inlet flow and pressure, it is no always possible to calculate the outlet pressure. If the flow is too high, the valve will be choked and in steady state it will not solve. If you specify the upstream and the downstream pressure, it will always solve for flow.
In HYSYS Dynamics, the valve has always defaulted to solving by using the valve opening and characteristics. If in a particular case you would want to force a fixed pressure drop, that is possible using the check box on the Dynamics Specs page. And in some special cases it may be useful to uncheck both of the check boxes there to give you the freedom to specify something special. Say that you wanted to bypass a fixed flow around an equipment. You could put in the TEE and Mixer and a valve in between and then add a flow controller to get the flow that you want. But if you wanted that flow to always be exactly what you wanted, you could make one of the streams in the bypass flow specified and uncheck both check boxes to give the solver the necessary degree of freedom.
HYSYS let's you specify Cg or Cv and if HYSYS detects a gas feed it will use the Cg by default, but noting stops you from flipping the radio button on the Rating / Sizing page back to Cv. Most people would feel more comfortable specifying a Cv as it is usually what you find on a data sheet. There is no problem using Cv for a gas valve, so fee free. HOWEVER, if you have a valve with a high pressure drop with the possibility that the flow goes critical, it is useful to consider what the Cg stands for:
Cg is a measure of the critical flow of a valve and C1 (Cg = C1 x Cv) will define at what pressure drop you will achieve critical flow. So, whether you use Cv or Cg, it will be good to also have a look at C1 and check at what pressure ratio you reach critical flow. The rule of thumb is that you reach critical flow is dP/Pin = 0.5. To match that closely, you need a C1 of 26.85, HYSYS has a default C1 of 25, which brings you pretty close.
You might be interested to know that when you switch the "Valve Manufacturer from "Universal Gas Sizing" to any of the manufacturers (not the simple k-value), you will get the option to use the ANSI/ISA formulation. In that formulation the relationship between Cg and Cv is: Cg = 40 x Sqrt(Xt) x Cv. The default value of Xt is 0.7, that makes a C1 of around 33. So, your valve will behave somewhat different when you switch to this formulation. 
You can find all this info in the HYSYS documentation and I also found the a PDF document from Emerson Process interesting.


Sunday, 5 January 2014

Modelling dynamic flare networks in dynamics - some tips&tricks

There is a lot of interest in modelling flare networks dynamically these days. Although the HYSYS Dynamics pipe segment doesn't model all phenomena rigorously, you can do a pretty good job with it. What the pipe segment is lacking is the momentum balance and it does't account for kinetic energy. So, you should use it to evaluate what happens in the really short term after opening a relief valve. I mean, not what exactly happens in the first couple of seconds. The lack of kinetic energy term will affect the temperature results, but the deviations shouldn't be too larger. I have no direct comparison of dynamic cases, but steady state cases indicate it should be within a couple of degrees C.

With that out of the way, there are a couple of practical things that are good to know when you start modelling a flare network in dynamics.

- HYSYS Steady State has no network solver (except Aspen Hydraulics), so it may be just as easy to start  in dynamics right away.
- Start with a case where you actually do have flow from all sources. It doesn't need to be the full flow, but some flow for sure.
- As the dynamic solver is more prone to failing once in a while, make sure you frequently save things.
- Build up the network a couple of pipes at a time.
- Do NOT try to put all the detail you have into the model. lump all the bends and tees and whatever else into a single fitting. A swage is a particularly nasty one to have. The problem being that a swage that enlarges the pipe diameter increases the pressure, so HYSYS Dynamics will translate that into a negative k-value and when your flow goes away from the flow where it was calculated, all sorts of odd things can happen.
- Use the full Churchill correlation on all pipes
- Do not forget to turn on the holdup calculations on all pipes, the default value is off.
- If you want to model the heat loss, best do so from the start. Make sure temperatures are behaving before you move on to model your next couple of pipes.

There may be a few more things to consider, I will add these as they come to mind, but the above ones should save you a lot of grief already.

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