Using the Dynamic Depressuring Utility To Simulate High Pressure Blowdown Scenarios

Over the past several years, Process Ecology has completed many plant-wide blowdown reviews of high pressure flare systems using the dynamic depressuring utility, which is available as part of the Aspen HYSYS® and Honeywell UniSim® software packages. Typically, the main objective of these reviews is to determine whether or not blowdown can be accomplished according to API standards (see section 5.20.1 of API 521) without exceeding maximum gas rates in the high pressure flare header. If it is anticipated that the maximum gas rate will be exceeded, it is desirable to modify the blowdown valves accordingly, or propose a staged blowdown strategy to accommodate the blowdown volumes. The depressuring utility can be used to perform a dynamic simulation of the blowdown event, and in turn provide depressuring and gas rate profiles used in the analysis. Other possible applications of the depressuring utility include PSV sizing studies as well as low temperature embrittlement studies (however, there are known limitations to using the depressuring utility for low temperature studies). Although dynamic in nature, the depressuring utility does not require a dynamic license to run and is included in the steady state package. The purpose of this article is to offer general advice and a few tips regarding the use of the depressuring utility.

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Depressuring Utility Results - Pressure (blue), Gas Rate (green), Temperature (red)

The depressuring utility can be used to simulate depressurization of vessels and pipelines, or the combination of several connected vessels and pipe volumes through a single valve. In general, the following information is required to perform blowdown calculations using the depressuring utility:

Identification and confirmation of each blowdown valve in the plant: an easy way to identify blowdown valves is to find the flare header in the plant P&IDs and carefully trace each process line upstream to the upstream equipment. It is important to ensure that ALL blowdown valves (BDVs) are included in the analysis (which may not be obvious, since not all the BDVs are always labelled as such).
Identification of shutdown valves: Identification of emergency shutdown valves, or valves with shutdown capability (which are a part of the blowdown philosophy), upstream and downstream of each blowdown valve on both liquid and vapour lines is required. The total volume of all equipment and piping between upstream and downstream shut down valves becomes the basis for blowdown calculations for that particular blowdown valve. If plant isometrics are not readily available, the plot plan should be used to estimate pipe lengths, while vessel dimensions and pipe diameters can be obtained from P&IDs. Piping volume can be a significant portion of the blowdown volume, particularly for smaller vessels, and should not be ignored.
Identification of all major plant equipment and piping: This step is important in order to ensure no major equipment is inadvertently left out of the review. It is a good idea to obtain the blowdown philosophy from the client (if available) as it may not be obvious which blowdown valve is responsible to protect a particular piece of equipment. If major pieces of equipment are unprotected, further review, and possibly design work, is needed.
Determination of valve type and trim sizes for each blowdown: If the objective is to review existing blowdown valves, it is important to confirm the valve type and trim size for each blowdown valve. With this information, valve parameters used in the depressuring utility (i.e.: Cv, C1) can be obtained from the applicable valve catalogue or technical bulletin for the corresponding valve.
Determination of blowdown volume: The blowdown volume for each valve is determined by summing the volumes of all vessels, as well as piping between shutdown valves upstream and downstream of the blowdown valve in question. As mentioned, the depressuring utility simulates the combination of several connected vessels and pipe volumes as a single vessel; this is a key assumption – if the user wishes to simulate individual vessels and pipes, they must use the full dynamic capability rather than the depressuring utility.

By default, the utility calculates the “flat end vessel volume” (volume of the cylindrical portion of the vessel without the heads) based on the height and diameter of the vessel entered by the user. We add the total vessel/piping volume (including vessel heads) into the utility and calculate the artificial height and diameter based on the L/D of the vessel with the largest volume being protected by the blowdown valve under consideration. Also, heat transfer areas of vessel heads are assumed flat circular; therefore, you may wish to consider replacing heat transfer areas calculated by the depressuring utility with heat transfer areas based on more acceptable vessel head geometry (e.g., spherical).

Configuration of Strip Charts: In our experience, the default strip chart tracks far too many parameters to be useful, and we recommend that the user create another strip chart with just the variables they want to view. If the user simply deletes variables from the default strip chart (containing “DL” in the title), a nasty surprise awaits them when the strip chart is rerun. All of the default parameters will be mysteriously reselected, and the strip chart will once again be a confusing mess.
Correction Factors: According to the Aspentech depressuring guide, the correction factors allow the user to account for the amount of metal in contact with the top or the bottom of the vessel as well as “additional nozzles, piping, strapping or support steelwork in close contact with the vessel”. This is equivalent to adding additional vessel mass to account for fittings and is used by the depressuring utility when performing the calculations. For the fire case, this extra metal mass will absorb a certain amount of fire-generated heat creating a slightly less conservative scenario. For a more conservative approach, the metal mass correction factors should not be specified.
Estimation of liquid holdup: The P&IDs may provide normal liquid levels (NLL) suitable for estimation of the liquid volumes; in some cases a reasonable estimate needs to be made (e.g., if the vessel is known to contain liquid holdup, one might assume it is 50% full). Liquid lines are assumed 100% full.

The reader should be aware of the limitations associated with the HYSYS® Depressuring utility. The purpose of a plant blowdown is to safely evacuate the inventory from process equipment according to accepted engineering standards (e.g., section 5.20.1 of API STD 521). It is desirable to blowdown process equipment in a reasonable period of time as dictated by the engineering standards and limitations of the flare header, in order to decrease the chances of damaging expensive equipment or injury to plant personnel. Rapid depressuring, however, may lead to another issue; equipment failure due to low temperature embrittlement. The HYSYS® dynamic depressuring utility is capable of calculating the temperature profile of the bulk process fluid temperature; however, it does not predict the temperature of the piping wall downstream of the blowdown valve. Bulk fluid temperatures are often cited in determining the material used to construct the blowdown flare header; however, bulk temperatures are often far cooler than the actual wall temperature of downstream piping. A more sophisticated (and accurate) approach, which would help predict equipment/piping wall temperature, is to perform calculations which incorporate heat transfer across the fluid boundary layer. Although much more rigorous, this approach could translate into significant cost savings if it meant construction could proceed using a cheaper low temperature carbon steel (LTCS) rather than Duplex stainless steel (DSS).

Another possible limitation of the HYSYS® depressuring utility mentioned earlier is that it models the entire volume of multiple vessels as a single vessel. It does not consider restrictions to flow via valves, orifices and fittings in piping between vessels.

Due to these limitations within the Depressuring utility, the user should apply careful analysis to ensure the depressuring is properly specified. In our experience, we have found the Depressuring utility to be very useful in the analysis of high pressure blowdown scenarios.

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