For most unit operations in HYSYS (Steady-State), pressure profiles and/or pressure changes in equipment are specified parameters. Often one needs to more accurately model pressure profile, such as when plant pressure drop is estimated in order to determine compression requirements. This leads to the following questions:
- What are reasonable estimates of pressure drop for process equipment?
- How can HYSYS update pressure profiles and/or pressure drops if conditions change?
Each of these questions is addressed below.
Estimating Pressure Drop
Many engineering and operating companies have their own guidelines in terms of setting appropriate pressure drops. Here we provide a few guidelines:
For gas pressure drop in exchangers (shell/tube, air cooled, double pipe), assume 1-2%
of operating pressure. Typically this should range from 3 to 10 psi (20-70 kPa).
For a better estimate (especially on the shell side of an exchanger), use a rigorous rating
program such as HTFS-TASC.
A pressure drop of 5 psi (35 kPa) is usually reasonable above moderate pressures (e.g.,
150 psi). For compressor scrubbers and associated piping, use a pressure drop of approximately 4 psi (30 kPa) for every 300 psia (2,075 kPa) working pressure up to 1,200 psia (8,300 kPa). Filters can introduce significant pressure drop; at higher pressures 10 psi is typical for dirty filters.
A pressure drop of 0.1 psi/tray is typical but can vary significantly, depending on tray type, liquid and vapour loads. Packed bed pressure drops are usually significantly smaller, and a preliminary estimate of 0.01 psi/ft (0.2 kPa/m) can be used.
The HYSYS Tray Sizing Utility can be used as a guide to estimate tray and packing pressure drops.
Piping pressure drop can vary widely; the HYSYS pipe sizing utility can be used to predict the pressure drop. Generally, piping pressure loss is less than 2 psi/100 ft. If piping pressure drop is included in the simulation, it either needs to be lumped into equipment pressure drop (e.g.,. at a separator), or alternatively modeled as a pipe or simply a valve with fixed pressure drop.
In many cases, the discharge pressure will simply be specified. However, compressor and/or pump curves (in the case of centrifugal compressors or pumps) can be used to provide the pressure-flow relationship.
There are many sources of information (the GPSA Handbook is one useful reference) which can help in providing guidelines; as mentioned before, engineering and operating companies often have their own guidelines for stimating pressure losses.
Updating Pressure Profile as conditions change
HYSYS users may wish to have pressure drops vary with process conditions (e.g., flow). This is possible using the HYSYS spreadsheet operation, which can calculate pressure drop as a function of imported conditions, then export the pressure drop to the corresponding equipment. As an example, the GPSA Handbook provides a function which allows the user to calculate relative pressure drops based on changes in conditions such as flowrate (1). In other cases, pressure drops may be updated automatically by HYSYS, such as when a heat exchanger is rigorously modeled (using the geometry) and pressure drops are calculated.
In other cases, the inlet pressure of a unit operation may change, and the user will want to ensure that the pressure profile is updated accordingly – generally this is easily accommodated since pressure drops, rather than absolute pressure, are usually modeled. The exception is with the HYSYS column, which requires a pressure profile as an input. A handy workaround in this case is to use “Set” operations to fix the product stream pressures as shown in the figure below. The pressure profile in the column (i.e.,
top and bottoms stage pressure specifications) would have to be deleted to allow the pressure from the product streams to set the pressure in the column. When the inlet pressure to the column changes, the column pressure profile would also be updated.
The effort that needs to be taken in setting up the pressure profile is a function of the objective for the HYSYS model. In many cases, the user does not need to worry about this aspect of the simulation.
However, when one needs this level of detail, HYSYS provides the flexibility to do it.