Common Over-Pressure Sources

External Fire

Fire is a common cause of over-pressure in pipes and vessels. Analyzing for fire case assumes that the vessel exposed to fire is blocked in.

If the vessel has some liquid inventory, liquid inside will absorb the heat through the vessel wall when it is exposed to an external fire. If the liquid inventory is low or if the content is a gas, the heat from external fire is not dissipated by vaporizing liquid. Fire protection is a complex topic covered by a multitude of codes. This is not covered here.

Blocked Outlets

The closure of a block valve on the gas outlet line of a pressure vessel can cause the vessel's internal pressure to rise and exceed its maximum allowable working pressure (MAWP).

For the case of blocked liquid outlet, the liquid level in the pressure vessel may rise. If the surge time for liquid overflow from the vessel is less than 15 - 20 minutes, then a blocked liquid outlet is a valid over-pressure scenario. If the surge time is longer, it can be assumed that operators have sufficient time to take action to avoid an overflow.

In the case of 2-phase mixture (such as the feed to a flash separator), the liquid level rising to a certain height may make separation impossible. The required relief capacity should also include the vapor.

Utility Failure

Unlike other scenarios that can only affect one vessel at a time, utility failures such as loss of power or cooling water can affect all equipment using power or cooling water at the same time. To be safe, the flare header should be designed and sized based on the maximum relief load that could result by a potential utility failure.

Power Failure

A general power failure affects the entire process plant and the immediate consequences are as follows:

• Electrical motor-driven pumps and compressors will stop. Thus feed input, product output and reflux flows will stop. Instrument air will be interrupted.
• Motor-driven fans for air coolers, cooling towers and combustion air blowers will stop. Cooling water supply will stop.
• Motor-operated valves (MOVs) will fail.
  

Use of UPS or emergency diesel generator may be considered so that when the normal power supply is interrupted, a standby power supply would automatically start, in a fraction of a second, to support critical equipment or units.

Due to the risk of power failure, API RP521 does not permit credit to be taken for pumps with standby or automatic startup, even when the pump has a separate power supply, because such a set up cannot be considered reliable in the event of utility failure.

Under partial power failure, the effect is limited to one process unit only. One can assume that the general utility supply would not be affected.

Instrument Air

Instrument air supply may have a large air reserve. This, and the high air pressure may, after a power failure, be able to maintain an adequate air supply for a short period of time. Normally the air receiver should be able to maintain 10 - 15 minutes of air supply after the air compressor has stopped. To be safe, it is prefer that credit is not taken unless the air supply is sufficient to last long enough for some corrective action to be taken.

Steam

Besides functioning as a heating source for reboilers, vaporizers, etc, as well as for stripping in fractionation columns, steam is also used as a driving force for pumps, turbines, compressors, blowers, etc. Loss of steam to these equipment will cause these equipment to stop working.

Fuel Gas and/or Fuel Oil

Loss of fuel gas and/or fuel oil would not generally cause over-pressure. Care should be taken in the case of fired heaters, especially when the heat duty is large, as the residual heat can cause over-pressure. Generally, 25 - 40% of normal heating duty shall be used as residual heat input once power fails and fuel gas or fuel oil input has stopped.

Cooling Water

When cooling water is used to condense vapors, care must be taken in relief calculations to consider the actual relief conditions, not the normal operating conditions. The required relief capacity must be equivalent to the total vapor load being condensed. At the over-pressure relief conditions, more vapor may actually become condensed because of higher pressure.

Loss of Cooling Duty

There are different types of cooling duties besides the loss of cooling water mentioned previously. In some processes, a quench stream is used to cool a vapor flow in a column, vessel or reactor, so the loss of quench stream means a loss of cooling duty. A loss of cold feed is similar to the loss of quench.

Likewise, a reflux functions as a quench stream. A loss of reflux causes the temperature in the column to rise, leading to vapor build-up and over-pressure of the column.

When an air-cooled exchangers fail, natural convection continues. Thus, a credit for 20 - 30% of normal duty can often be taken to reduce the required relief capacity. If the louver is closed, then the failure is to be considered as total loss of cooling duty.

Thermal Expansion

It is rare that a vapor-filled line or vessel would be over-pressured by external heat source, other than fire. On the other hand, when a liquid is blocked in a vessel or pipeline, external heat input can cause liquid temperature, and hence volume, to rise. Common causes are:

• The pipeline is filled with liquid that is blocked in and heated by steam or electrical heat tracing
• A heat exchanger cold side is filled while the hot side is still flowing
• A pipeline or vessel is filled with liquid at ambient temperature, and is heated by direct solar radiation
  

When the liquid being blocked in has a vapor pressure higher than the relief valve set pressure, thermal vaporization is a potential cause of over-pressure.

Abnormal Heat Input

Some possibilities include:

• The supply of heating medium, such as fuel oil or fuel gas to a fired heater, can be switched from medium heating value to higher heating value
• Heat transfer in new and clean heat exchanger after a revamp
• The control valve for the fuel supply fails fully open
• The supply pressure of the heating steam is changed from its normal range to its maximum pressure

Abnormal Vapor Input

This may occur when the upstream control valve fails fully open. This is especially important if the upstream equipment is under high pressure. The system may not be able to discharge the extra vapor from the upstream process and eventually become over-pressured.

Loss of Absorbent Flow

In general, loss of absorbent will not result in an over-pressure situation. However, when gas removal by an absorbent is more than 25% of the total inlet vapor flow, an interruption of absorbent flow could cause pressure to rise in the absorber.

Entrance of Highly Volatile Material

An example would be water or light hydrocarbon entering into hot oil during a process upset. The vaporization causes an instantaneous phase expansion which can be enormous.

Accumulation of Non-Condensibles

During normal operations, non-condensibles do not accumulate in the system, because they are discharged along with the process streams or through a vent. Accumulation may occur under the following conditions:

• The normal non-condensible vent is blocked
• The piping configuration or equipment has a pocket in which non-condensibles accumulates

The accumulated non-condensibles can blanket a condenser and result in loss of cooling duty.

Valve Malfunction

Only one valve malfunction needs to be considered at a time, if other valve malfunctions are unrelated. Examples of valve malfunction include:

• Check valve backflow - though this is a rare case, but should be considered when a high pressure differential is present between the inlet and outlet of a check valve, a pump or other piece of equipment.
• Inadvertent valve operation (open/close, bypass) - a valve may be wrongly operated to a position that is opposite its intended normal operating position. This is a very common over-pressure scenario, and is the result of human error. Very often, a fully open bypass valve can pass a greater fluid volume than the main control valve.
• Control valve fails fully open or closed - caused by mechanical or electronic signal failures.

API RP520 and 521 do allow one to take credit for use of "car seals open" (CSO) valves. Such valves maintain an open position all the time. The reverse is the "car seals close" (CSC) valves that are in close position all the time.

Exchanger Tube Rupture

API states that complete tube rupture, in which a large quantity of high-pressure fluid will flow to the low-pressure side of the exchanger, is a remote but possible contingency.

Upstream Relieving

The following scenarios should be evaluated:

• When an upstream vessel is relieving by discharging fluid into a downstream vessel, the downstream vessel must be designed to handle the pressure and volume of the incoming stream from the upstream vessel without over-pressuring itself. If the upstream vessel does not have adequate relief capacity, the downstream vessel needs to have a pressure relief valve of its own.
• 2 vessels are connected by an open path. The first has its own pressure relief valve and it discharges into a flare header. The second will experience the impact from the relieving pressure of the first vessel relieving.

Runaway Chemical Reactions

During runaway reactions, which tend to accelerate with rising temperature, extremely high volumes of non-condensibles with high energy can cause the internal pressure of a vessel or pipeline to rise rapidly. Basically, pressure relief valves may not provide adequate protection at all because of their relatively slow response time. In such a situation, vapor depressuring systems, rupture discs and emergency vents are preferred.

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