The Lowdown on Blowdown

Chris Best addresses the role blowdown systems play in plant design and why they are essential in order to ensure process safety

TO ensure the safe operations of refineries and petrochemical facilities, emergency vapour depressurising (blowdown) is a standard practice. Its aim is to reduce the failure potential of a vessel by reducing the inventory and therefore pressure within the system.

This is particularly relevant in the event of overheating through scenarios such as external pool fires or runaway reactions. During prolonged heat exposure, metal temperatures can increase such that stress rupture can occur, even if the vessel remains within its maximum allowable accumulation. The act of blowdown during a fire emergency ensures that the vessel’s internal stress is reduced, extending the vessel life at the elevated temperatures.

The anatomy of a blowdown system

A typical blowdown system will isolate a section of plant at the same design pressure through the use of emergency shutdown valves. The path to depressurise the process inventory is usually routed through a restriction orifice, although a control valve is sometimes used, to control the peak flowrate to the flare stack. This is important, as the flare will have a finite capacity and hence this cannot be exceeded without risk of mechanical damage to the flare tip or piping. A typical process train will be made up of a series of individual blowdown systems that allow the systematic depressurising to safe inventories levels. Blowdown systems are segregated for several reasons, including allowing staggered blowdown (to minimise flare capacity), targeted blowdown (to allow blowdown of a specific system where gas detection or fire detection has been activated) and because the plant may have multiple design pressures, hence a single blowdown system is not acceptable.

Conforming to safety regulations

Blowdown systems are typically sized for the fire scenario, with guidance from API521 identifying a timeframe of 15 minutes to take the system from initial conditions to 50% of the design pressure. The basis for this guidance is ensuring the exposed wall temperature does not exceed the stress to rupture, specifically for carbon steel vessels with a wall thickness of 1” (25.4 mm).

For installations with multiple, large vessels operating at high pressures, the peak demand to the disposal system can be an unrealistic sizing basis considering the concurrent rates required to meet the outlined timeframe. Sizing a flare system to deal with this transient peak rate is often cost prohibitive and the use of staggered blowdown can yield a more cost-effective solution. In addition, large vessels operating at high pressure may have wall thicknesses of greater than 1”, therefore the vessel survivability can be explored further to provide a wider blowdown window, while not exceeding the stresses that can lead to vessel rupture, in the event of a fire.

Conversely, systems containing vessels with a wall thickness below 1” would require further assessment to ensure that the blowdown occurs proportionally faster than the 15 minute guidance as their internal stresses could reach failure point more quickly.

In the majority of scenarios, the peak blowdown rate is determined by the fire case and the minimum design metal temperature is defined by the cold case

The secondary functionality of the blowdown system is to allow the system to be de-inventoried as rapidly as possible should a loss of containment be detected. The auto-refrigeration of the liquid phase contributes to low temperature within the vessel. This auto-refrigeration and subsequent Joule-Thomson effect of the vapour phase contribute to low temperatures within the blowdown outlet pipework. This scenario is often assessed at minimum ambient temperatures to provide worst “cold case” results.

In the majority of scenarios, the peak blowdown rate is determined by the fire case and the minimum design metal temperature is defined by the cold case. These two cases are often influenced by opposing factors, an example of this would be the inventory of liquid in the system. For the fire case, the greater the volume of liquid, the higher the wetted area and therefore heat input to the system. This generates more vapour to be removed from the system in the allotted timeframe and yields higher peak rates to flare. Conversely, the greater the liquid inventory during the cold case, the greater the heat sink during depressurising, resulting in warmer metal temperatures when compared to lower liquid inventory cases.

Care must be taken to avoid a catch-all sizing basis for both the fire and cold cases as what is conservative for one may not necessarily be conservative for the other.

Initial considerations

The starting conditions for assessing blowdown scenarios can be subjective, given the guidance available. The timer for the guideline of 15 minutes starts on detection of a fire but even when blowdown is automatically triggered there can be inherent delays. Often, safety systems will not open the route to blowdown until the system is confirmed to be isolated by the closure of these ESD valves therefore reducing the time available. This can be mitigated by considering the heat input of the fire and starting the blowdown at a more conservative pressure and temperature than what would be considered within the band of normal operating conditions.

Care must be taken to avoid a catch-all sizing basis for both the fire and cold cases, as what is conservative for one may not necessarily be conservative for the other

For cold blowdown cases it is important to review the initial conditions from “normal” operating conditions but also factor impact of minimum ambient temperatures should any delay to depressurising occur.

For systems that contain a pressure differential in normal operations, the isolated settle-out pressure should be assessed for both the fire and cold cases. A common example of this would be compression trains but it is equally applicable to systems containing stay-put control valves (eg a high-pressure separator and manifold within an offshore platform).  Calculation of the settle-out conditions will not only provide accurate starting conditions but will also ensure an accurate representation of the vapour composition at those conditions.

Developing an effective model

Simulation of blowdown can be carried out within various programs and they all have their own strengths and weaknesses. Simple blowdown systems consisting of a single vessel with nominal lengths of pipework can be modelled easily with a high confidence in the results. More complex systems, like compression trains or gas conditioning packages, can really test the constraints of the simulations used. Systems that comprise multiple vessels, with different wall thicknesses and geometry, can be particularly problematic.

Often, complex systems will be simplified to fit the available inputs of the simulation package in use. This approach must be treated with caution as it can mask potential issues. An example of this would be grouping of vessels with different phases present as a single simulated entity.  As mentioned previously, any liquid would act as a heat sink during the cold case, the model results would therefore return warmer temperatures when compared to modelling a vapour-only system.

Blowdown on existing systems

Over time, modifications may be made to the equipment or controls within the process plant which would directly impact the ability to blowdown compliantly. These modifications, such as new tie-ins, increases to system inventory or change in composition from an ageing field, will all have an impact on the blowdown design. 

A blowdown system is complex in detail and usually requires specialist analysis. It is therefore essential that the blowdown system is regularly reviewed to ensure that the system integrity, the sizing basis and potential loads placed upon it still fit within the original design envelope.