Usually when designing a process unit, steam grid operating- and design conditions will be a given fact. This is because most projects will be executed on a brown-field site where the utilities are already available (save for sufficient capacity maybe). But what if you are executing a green-field study for a new facility that requires a heating medium and steam is one of the options to be considered (as it often is)? How do you define the grid operating and design consitions then?

Temperature requirements

One of the first things to look at of course is what the process requirements are. You will need to know at what temperature levels the potential steam users operate to determine the minimum saturation temperature required in your steam heaters or reboilers. In addition you will need to make assumptions on a reasonable driving force (LMTD) for these exchangers in order to obtain reasonable sizing. This will then set the minimum steam side operating pressure in these equipment items. You will then need to allow for operating margin (usually 10% or 0.5 bar), pressure drop for control, and finally grid linelosses to get at the grid maximum operating pressure that corresponds with the user requirements.

On the other side you may have (potential) steam producers in your process. There the system works the other way around. You will need to assess the minimum steam side operating pressure (usually at turndown where you will require a lower LMTD than at full rates), subtract operating margin, control pressure drop for control, and finally grid linelosses to get at the the grid maximum operating pressure  that corresponds with the producer requirements.

Based on this evaluation you will be able to assess if you can use one steam level or of two or more would be appropriate. For instance if you have a producer that would need a 20 bar header to discharge in at turndown while you have a user that requires 30 bar at full capacity, you will obviously need two grid levels.

Steam balance

Besides the required temperature levels it is also wise to have a look at the steam balance in terms of the steam rates in- and out for each of the steam levels. This will provide insight in additional requirements like boilers or (condensing) turbines to provide or consume steam.

Pipe classes

Finally you will need to look at the natural boundaries that are presented by the standard pipe classes. These will be marginally different per client or site but in general they will behave similarly to the ANSI flange ratings.

As the required steam temperature increases, so will its saturation temperature and required operating temperature (saturation + superheat). At a certain point the combination of design temperature and design pressure associated with the required operating conditions will exceed the flange rating for a 150# system. This means that investment cost will make a step change here as we will need to design for 300# beyond that operating temperature. At a higher temperature still we will exceed the 300# limits and move into a 600# or even 900# design.

The following graph shows this effect based on typical values for operating and design margins:

Steam Condions vs Flange Rating

Roughly speaking a  150# system (in carbon steel) will allow up to 10 barg operating pressure, a 300# system allows 30 barg while 600# and 900# go up to 60 and 90 barg respectively. Corresponding grid design pressures (as shown in the above graph) will be 10-20% above these numbers.

Bearing these “natural boundaries” in mind, you can make better desicions on the allowance for driving force (LMTD) that you allow for in your heat exchanger design. Being penny wise here can turn out to be pound foolish if you’d have to increase the pound rating of an entire grid level as a result.

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