Combustible Dust Testing

Laboratory testing to quantify dust explosion & reactivity hazards

Flammable Gas & Vapor Testing

Laboratory testing to quantify explosion hazards for vapor and gas mixtures

Chemical Reactivity Testing

Laboratory testing to quantify reactive chemical hazards, including the possibility of material incompatibility, instability, and runaway chemical reactions

DIERS Methodology

Design emergency pressure relief systems to mitigate the consequences of unwanted chemical reactivity and account for two-phase flow using the right tools and methods

Deflagrations (Dust/Vapor/Gas)

Properly size pressure relief vents to protect your processes from dust, vapor, and gas explosions

Effluent Handling

Pressure relief sizing is just the first step and it is critical to safety handle the effluent discharge from an overpressure event

Thermal Stability

Safe storage or processing requires an understanding of the possible hazards associated with sensitivity to variations in temperature


Classification of hazardous materials subject to shipping and storage regulations

Safety Data Sheets

Develop critical safety data for inclusion in SDS documents


Model transport of airborne virus aerosols to guide safe operations and ventilation upgrades


Model transport of contamination for source term and leak path factor analysis

Fire Analysis

Model transport of heat and smoke for fire analysis

Flammable or Toxic Gas

transport of flammable or toxic gas during a process upset

OSS consulting, adiabatic & reaction calorimetry and consulting

Onsite safety studies can help identify explosibility and chemical reaction hazards so that appropriate testing, simulations, or calculations are identified to support safe scale up

Mechanical, Piping, and Electrical

Engineering and testing to support safe plant operations and develop solutions to problems in heat transfer, fluid flow, electric power systems

Battery Safety

Testing to support safe design of batteries and electrical power backup facilities particularly to satisfy UL9540a ed.4

Hydrogen Safety

Testing and consulting on the explosion risks associated with devices and processes which use or produce hydrogen

Spent Fuel

Safety analysis for packaging, transport, and storage of spent nuclear fuel

Decommissioning, Decontamination and Remediation (DD&R)

Safety analysis to underpin decommissioning process at facilities which have produced or used radioactive nuclear materials

Laboratory Testing & Software Capabilities

Bespoke testing and modeling services to validate analysis of DD&R processes

Nuclear Overview

Our Nuclear Services Group is recognized for comprehensive evaluations to help commercial nuclear power plants operate efficiently and stay compliant.

Severe Accident Analysis and Risk Assessment

Expert analysis of possible risk and consequences from nuclear plant accidents

Thermal Hydraulics

Testing and analysis to ensure that critical equipment will operate under adverse environmental conditions

Environmental Qualification (EQ) and Equipment Survivability (ES)

Testing and analysis to ensure that critical equipment will operate under adverse environmental conditions

Laboratory Testing & Software Capabilities

Testing and modeling services to support resolution of emergent safety issues at a power plant

Adiabatic safety calorimeters (ARSST and VSP2)

Low thermal inertial adiabatic calorimeters specially designed to provide directly scalable data that are critical to safe process design

Other Lab Equipment (DSC/ARC supplies, CPA, C80, Super Stirrer)

Products and equipment for the process safety or process development laboratory


Software for emergency relief system design to ensure safe processing of reactive chemicals, including consideration of two-phase flow and runaway chemical reactions


Facility modeling software mechanistically tracks transport of heat, gasses, vapors, and aerosols for safety analysis of multi-room facilities


Our highly experienced team keeps you up-to-date on the latest process safety developments.

Process Safety Newsletter

Stay informed with our quarterly Process Safety Newsletters sharing topical articles and practical advice.


With over 40 years of industry expertise, we have a wealth of process safety knowledge to share.

Recent Posts

Pressure Locking of Safety Related Valves

Posted by Chris Henry, PhD on 01.11.19

By Chris Henry, PhD., Senior Consulting Engineer, Fauske & Associates, LLC

Pressure locking of safety related valves has been discussed recently in Nuclear Regulatory (NRC) Information Notices 95-14 and 95-18. Pressure locking may occur in gate valves if a water-solid valve bonnet is heated. Valve bonnets become water-solid due to normal cycling, or a leak created when the disc moves away from its seat. Even a modest water temperature rise (10 to 15ΕF) can greatly increase the bonnet pressure and cause a pressure differential that the valve actuator cannot overcome. This would prevent the valve from opening and performing its safety related function.

An example outlined in NRC Information Notice 95-14 for pressurized water reactors relates to the containment sump(s). Pipes from the sump to low pressure injection suction typically contain two normally-closed motor-operated gate valves, as shown by Figure 1.

Pressurized water reactor

Figure 1

Both Motor Operated Valves (MOV)s must open during a design-basis Loss Of Coolant Accident (LOCA) to provide containment sump water for Reactor Pressure Vessel (RPV) injection and containment spray recirculation. The second valve (from the containment sump) has Refueling Water Storage Tank (RWST) water at a static pressure of, say, 35 psig on one side. If the valve disc in this second MOV leaks and RWST water fills the bonnet, pressure lock could keep the valve from performing its function during a DBA LOCA. Assuming a water-solid bonnet, the question then is whether the valve bonnet temperature rise will create pressure lock before the MOV must open for RPV and containment spray recirculation.

In a water-solid system, pressure would rise 55 psi for every 1 degree F increase in temperature. The valve bonnet may not be truly water-solid, owing to leaks, but the 55 psi/F value is conservative. The temperature rise of the bonnet water depends in turn on the thermal boundary conditions that the valve must operate under. During a (Design Basis Accident) DBA LOCA, two heat sources might affect the second MOV: (1) hot water in the containment sump, and, (2) the valve room ambient conditions, given that the containment spray pump operates in the same room as the valve and heats it to 120°F within 30 minutes.

A realistic thermal analysis of the valve and bonnet water is required to judge the potential for pressure lock in a DBA LOCA. In addition to the boundary conditions above, the analysis is also constrained by the time to recirculation switchover – about 30 minutes if all safeguards operate. First consider the water heatup due to the rise in room temperature. The valve acts as a barrier between the bonnet and the room and provides "thermal lag" for the bonnet water. Since thin metal heat sinks have low Biot numbers, the time constant for valve heating, τ, is given by:

τ = ρτc

where ρ is density of carbon steel, t is the metal thickness, c is the specific heat of carbon steel, and h is the natural convection heat transfer coefficient. Representative values are: ρ = 500 lbm/ft3, t = ½", c = 0.1 Btu/lbm-F, and h = 1.0 Btu/ft2-hr-F. These values result in a time constant of 2 hrs, which suggests that the room temperature rise is unnoticeable to the bonnet water within the 30 minute time to recirculation switchover.

Heating due to the containment sump water is more complicated, mainly because the sump water temperature boundary conditions is a function of time. Figure 2 shows sump water temperature from Modular Accident Analysis Program (MAAP) 3.0B code calculations for a DBA LOCA in a large, dry (Pressurized Water Reactor (PWR) containment with all engineered safeguards available. The initial sump water temperature is very high, as primary system water flows through the break onto the containment floor. But containment sprays and fan coolers add colder water to the containment floor pool, and the average pool (sump water) temperature decreases. A key phenomenological question is how quickly the hot water in the pipe length from the sump to the first MOV mixes with colder overlying water from fan cooler condensate and containment sprays. If the first valve is exposed to the average pool conditions, rather than hot water trapped in the pipe length, the heat load to both MOVs is reduced.

Sump Water Temperature

Figure 2

This is essentially a problem of counter-current flow in the pipe length between the sump and the first MOV. The counter-current flowrate can be estimated by:

Wcc= 0.15√g(ρH - ρc) ρc D5

where g is the acceleration of gravity, ρH is the hot water density, ρc is the cold water density, and D is the pipe diameter. This flowrate gives an estimate of how quickly hot water in the pipe length will mix with cold water in the sump.

Assuming a 14 inch diameter pipe, hot saturated water at 240°F, and cold saturated water at 220°F, ρc = 59.08 lbm/ft3, ρH = 59.61 lbm/ft3, and the counter-current flowrate equals 7 lbm/s. If the pipe is 30 ft long, cold water in the containment sump would flush out the hot water in the pipe in about 3 minutes. Therefore, the containment sump water temperature as predicted by MAAP 3.0B is an appropriate boundary condition for the time frames much longer than 3 minutes.

A conduction analysis of the pipe length between the first and second MOVs is required, with the sump water temperature as the boundary condition at the first MOV, and, say, an adiabatic boundary condition at the end of the second MOV. The temperature distribution along the pipe can be modeled by a steady-state solution to a fin equation. A steady-state solution is conservative and readily expressed in analytical form. The temperature distribution is then governed by the pipe conductivity and the heat transfer coefficient from the pipe to the surroundings. For the conditions discussed above, a fin equation solution shows the bonnet water temperature rise would be less than 1°F.

The basic thermal analysis shown above demonstrates that in this example, pressure lock is not likely to prevent MOV operation within the 30 minute time of recirculation switchover. Nevertheless, given the bonnet water temperature rise, a final water pressure can be calculated using the 55 psi/F value, and compared against the thrust developed by the MOV actuator. Conclusions drawn here depend very much on the 30 minutes time to recirculation switchover. Results of the thermal analysis might differ if the time to recirculation switchover is increased due to a postulated equipment malfunction or operator error. Other potential pressure lock cases may require similar review for event timing and thermal boundary conditions.

Fauske & Associate, LLC, world leading process safety consultants, provides expert, custom, full-service safety testing, engineering, analytics, consulting and training solutions to nearly every industry. We use a data-backed approach to solve complex process safety problems and mitigate severe accidents. Request a quote today.

Contact Us


Topics: process safety, pressure relief, nuclear


Is My Dust Combustible?

A Flowchart To Help You Decide
Download Now