Solenoid Valves for Methane Emission Reduction

Hydraulic fracturing and horizontal drilling have opened up vast new areas for low-cost natural gas production, and consequently have altered the energy landscape in the United States.  The rush to develop this new resource via new techniques has resulted in numerous environmental challenges, including water and air quality concerns.  Preventable emissions of methane, a potent greenhouse gas with approximately 30 times the heat trapping ability of CO2, are cited by some as among the easiest of those challenges to address.  According to the U.S. Energy Information Administration, marketed production of natural gas increased from approximately 1,650,000 to 2,440,0000 million cubic feet between 2005 and 2017, as in projected to continue the rise.

Methane leaks occur during the production, processing, and transmission of natural gas.  Pneumatic devices, powered by natural gas under pressure, regulate various aspects of the gas passing through them, including temperature, pressure, and flow rate. Many pneumatics are powered by natural gas under high pressure, and vent (or “bleed”) some of that gas to the atmosphere as part of normal operations. High-bleed pneumatic devices are a significant source of methane emissions throughout the supply chain.  According to EPA Inventory data, in 2013, pneumatics emitted roughly 638,000 metric tons of methane, over 20 million metric tons of CO2 equivalent, or a full one-third of all methane emissions from the production sector.  Other studies exist that suggest this might be underestimated by over 40 percent.  Replacing or retrofitting a continuously or intermittently emitting high-bleed controller, defined as emitting an average of over six standard cubic feet (scf) per hour as part of normal operations, can make economic sense.

An alternative solution that eliminates methane emissions altogether is to replace gas-driven pneumatic devices with ones that rely on electricity instead – solenoid actuated valves.  Solenoid valves can be purchased from manufactures such as Clark Cooper configured with certified explosion proof coils per UL-1203 and CSA C22.2 #30.  Stainless steel valves are readily available in sizes 1/2” through 2” with maximum allowable inlet pressures of up to 1500 psi.  Where a power grid is not available, DC coils are available that can be battery operated.  External leakage is essentially zero.  An expensive compressed air system is not required.

How does a Solenoid Valve work?

A solenoid, as it pertains to a valve, is an electromagnetic device consisting of a coil of insulated wire and an iron-based permeable plunger and pole piece.  The following sequence demonstrates how such an arrangement can be utilized to make a plunger work to open or close a valve.  Such is called a “solenoid valve”.

 

Figure 1 is a section view of a piloted, floating piston, high pressure solenoid valve.  Pressurized fluids enter at the inlet and are held stationary until the piston moves away from the orifice.

A coil is shown surrounding a pole piece outside of the pressure boundary.  The plunger is a free floating piece of permeable steel.

 

Figure 1 – Nomenclature

In Figure 2 the valve is in the closed state, meaning that the piston is completely blocking the orifice.  The pilot hole through the center of the piston is also blocked by a pilot pin that is forcibly held down by a compression spring.  Fluid pressure exists at the inlet and in all areas highlighted by the red boxes.  No power is being delivered to the coil.

The valve shown is a “normally closed” or “fail closed” valve configuration, which means that if the coil is not energized, the valve will be in the closed state.

The pressure differential that exists between the inlet and outlet creates a force on the piston and pilot pin as well.  Higher pressure differentials equate to higher forces sealing the valve.

Figure 2 – Closed

 

 

Voltage is applied to the coil in Figure 3.  The iron-based plunger and pole piece become magnetic, and that force draws them together quickly.

In Figure 3, it is shown that the plunger has moved upward.  However, the valve is still closed.

Figure 3 – Plunger initial movement

 

The coil remains energized in Figure 4.  The magnetized plunger has now made contact with the pole piece and has pulled the pilot pin away from the pilot orifice.

Fluid has suddenly been allowed to flow through the tiny pilot orifice through the center of the piston.  As a result, the pressure has suddenly dropped directly above the piston, represented by the green box.

There is now a net upward force on the piston due to different pressures acting on differently sized areas of the piston.

 

Figure 4 – Pilot orifice open

The net upward force on the piston quickly pops it away from the orifice and fully opens the valve.

Red boxes in Figure 5 represent where pressures are approximately equal to the inlet.

A blue box represents downstream pressure, which is slightly less due to flow friction.

When the coil is turned off, the plunger and pole piece go back to a non-magnetic state.  The compression spring pushes down on the pilot pin, which fully blocks the pilot orifice.  A small bleed hole in the side of the piston allows fluid to re-enter above the piston.  A net downward force is generated due to the new pressure imbalance and the valve quickly closes.

Figure 5 – Piston moved

Valve Body Connections for High Pressure Solenoid Valves

NPT

NPT American National Standard Taper Pipe Threads per ANSI/ASME B1.20.1 are commonly used on high pressure solenoid valves.  The maximum allowable pressure is dependent on size and connector material, but can be as high as 10,000 psi.  It is therefore always prudent to investigate pressure capacities with the component manufacturers.

Picture 1 – ¼” NPT

Female NPT threads are created in valve bodies by tapping, single point turning, or thread milling.  Unlike the SAE J1926 and Medium Pressure Connection, sealing is not created on interfacing smooth cone faces, but rather on the threads themselves.  In order to provide good sealing across multiple brands of connectors, female connections should not only meet the “L1” gage distance per ASME B1.20.1, but also provide an adequate number of full threads past the hand tight engagement length.

Application of a thread sealant is mandatory in order to block the spiral path between the peaks and valleys of the male and female interface of ASME B1.20.1 NPT threads.  Three to four wraps of PTFE tape is preferred over liquid sealants.  This is because overuse of a liquid may cause drippage into operationally sensitive areas of the solenoid valve.  Picture 2 is an example of how properly applied PTFE tape will look after a connector is removed from quality threads that have been fully tightened.  The white tape is evenly compressed across the faces and valleys.  One indication of improperly machined threads or inadequate length of engagement would be a thick string of PTFE nestled in the thread valley.  This is one clue that the components did not (or could not) compress together fully.

Picture 2 – Good application of PTFE tape

Installation torques are not commonly published for NPT connections.  It is somewhat of a trial and error system to get them to seal by keeping track of the angle of rotation past hand tight.  Reversing the connector out will ruin the sealant and will require the installer to re-apply.  Consult the specification for the number of threads that should engage, hand tight, for a given NPT size.

NPT is the easiest connection to manufacture, and is sufficient for the bulk of solenoid valve applications.  However, once an assembler gets the connection to seal, there is very limited flexibility on the angle of rotation to which it must connect to other things (e.g. piping, tubing, etc.) unless accommodations are made via adjustable tube connectors.  There are also minor nuances between connectors from different manufactures.

SAE J1926

Connections per SAE J1926/1 utilize straight ISO 725 threads (inch) and a smooth 12 or 15 degree sealing cone interface.  The male SAE connector has an o-ring of chosen material that seals on the female cone.  The specification states a maximum allowable pressure of 63 MPa [9135 psi] for connections put together with nonadjustable stud ends, but always be sure to check with the component manufacturers.

As seen in Picture 3, the SAE connection is somewhat more elaborate than the NPT.  For a comparable inner diameter through which fluid can flow, the connection tends to need more axial length in order to fit it into the valve body.

Picture 3 – SAE J1926-1 Size ‘-4’ Connection

From our experience at Clark Cooper it is easier to make a perfect seal with SAE connectors as compared to NPT.  Installation torques are readily accessible for SAE.  However, an end user must take the o-ring material selection into account to ensure compatibility with the fluid type and temperature.  The SAE connection is generally not used for cryogenic applications due to that o-ring.

MPF

The 20,000 psi “Medium Pressure Fitting” (MPF) connection utilizes inch-based standard thread and a 60 degree cone at the base of that thread.  All sealing is accomplished on the metal-to- metal cones without the use of an o-ring or thread sealant.  This versatile connection, despite not requiring an elastomeric o-ring, can still perfectly seal high pressure gases such as hydrogen and helium.

Care must be taken during machining to ensure a smooth cone.  Cleanliness is important as well during installation.

Picture 4 – LF6 Medium Pressure Connection

Solenoid Valve Certifications: What is CRN Certification?

CRN

CRN is an acronym for Canadian Registration Number, which is assigned by each province or territory of Canada to accept and register the design of a boiler, pressure vessel, or fitting.  The numbers after the decimal place in the CRN represent different provinces or territories as follows:

1 = British Columbia
2 = Alberta
3 = Saskatchewan
4 = Manitoba
5 = Ontario
6 = Quebec
7 = New Brunswick
8 = Nova Scotia
9 = Prince Edward Island
0 = Newfoundland
N = Nunavut
T = Northwest Territories
Y = Yukon Territory Purpose

A solenoid valve, for example, will be registered as a Type ‘C’ fitting.  The manufacturer would submit and demonstrate compliance to pertinent design standards, such as the ASME Boiler & Pressure Vessel Code via calculations, drawings, test data, etc.  A manufacturer must present proof of being regulated by a Quality Control Program such as ISO 9001 that is audited by a reputable authority.

The overall intent of the CRN is to give the end user peace of mind that the product is well designed and safe.  The vast majority of Clark Cooper Valves, for example, come with CRN approval.

UL-429

UL-429 is the Standard for Safety for Electrically Operated Valves. It includes requirements for various types of valves, such as valves for general purpose, hazardous fluids and designated safety.  A manufacturer must submit component drawings for the valves undergoing certifications, as well as the fluid type, fluid temperature range, and ambient temperature range for which the certification is to apply.

All valves must operate with fully heated components using 10% over nominal voltage, and just 85% of rated voltage.  External leakage, seat leakage and endurance tests potentially up to 100,000 cycles are included.

A certification to UL-429 provides an end user with 3rd party verification that a valve design was scrutinized rigorously through actual testing.

UL-1203

UL-1203 is the Standard for Safety for Explosion-Proof and Dust-Ignition-Proof Electrical Equipment for Use in Hazardous Locations and applies to the U.S.  In terms of equipment certification for hazardous location and explosion atmospheres, it defines the rules for a “protection type”.  One can view the Class, Division, Group, and temperature class description by downloading a guide summary.

Using the guide, the certification code can be broken so that an end user fully understands:

  • What gases can be safely used around the equipment
  • How often those flammable gases are present
  • The allowable surrounding ambient air temperature range
  • The maximum surface temperature that the equipment may reach

For example, a company may advertise “Explosion-Proof” per UL-1203, Class I, Division 1, Groups A through D hazardous locations with operating temperature code, “T2D”, for an ambient temperature range of -20 to +85 C.  Class 1 refers to flammable gases.  Groups A through D include certain gases such as hydrogen and acetylene.  T2D means that the surface temperature of the equipment will not exceed 215 C [419°F] in 85 C surrounding air.

CSA C22.2 #139

The CSA group, Canadian Standards Association, issued CSA C22.2 #139 as essentially the Canadian version of UL-429.  Like UL-429, a certification to this standard provides the end user with 3rd party verification that a valve design was scrutinized rigorously through actual testing.  It includes similar temperature, operation, endurance and leakage tests as UL-429.

ATEX

ATEX Certification of a solenoid valve implies conformity to European Directive 2014/34/EU for potentially explosive atmospheres per BS EN 60079-0 and 60079-1.  It is the European version of UL-1203, and similarly uses a code scheme.

The ATEX code will include equipment group and category, atmosphere group, protection type, and temperature class for a given ambient and fluid temperature range.

Certain valves manufactured by Clark Cooper are available with ATEX coils as well.

The Main Uses of Solenoid Valves and the Industries that Use Them

What Solenoid Valves Do

Solenoid valves are an advantageous solution for controlling the flow of many liquids and gases of a huge temperature range where either an “on” or “off” state is needed.  A piston that may contain a metal or soft disk either covers or moves away from an internal orifice separating the inlet from outlet.  When the piston is blocking the orifice, a pressurized fluid is held stationary at the inlet and up into the bonnet tube.  A differential pressure exists over the orifice area, for which the resultant force combines with any downward spring force to create a tight seal.  Upon energization of a solenoid, internal components move in various ways that result in the piston either moving away from or towards that orifice to open or close the valve.  If the valve was a “proportional” solenoid valve, essentially infinite positions of that piston could be created by varying the supply voltage to create a throttling effect. oriffice

Simple Setup

Solenoid valves are easy to install.  Many simply have two lead wires connected to a coil nested around a bonnet tube containing materials that easily magnetize and demagnetize.  Either wire can be line voltage.  Switches or relays can be designed into the control system to route power when needed to quickly open or close the solenoid valves.  Selection of a solenoid valve can eliminate the need for long and expensive air lines and air delivery systems that would be needed for pneumatically actuated valves.  For long wire runs on remote locations, higher AC voltages may be utilized to minimize power loss.  For example, 240V AC is commonly sold.

Voltages

Solenoid coils are offered in many voltages, typically 12, 24, 120, 240V DC, and 24, 120, and 240V AC, which suit the majority of applications. Coils are easily developed for less common voltages as well. While the valve may be designed to achieve catalog pressures assuming the nominal voltage, the valve may still operate effectively at voltages lower than those nominal values.  This feature is ideal for battery powered systems that may provide lower voltage over time.  An AC coil will generally run hotter than a DC coil wound for the same power output due to hysteresis characteristics of the magnetic circuit.  An AC coil will also have an initially high “inrush” of current on the order of 2-10 times the nominal steady state current.  A system designer should be aware of inrush amperage and heat outputs, along with the resulting coil surface temperatures.  These temperatures are dependent upon the duty cycle of the valve, which includes energization frequency and the duration held energized, along with ambient and fluid temperatures.  While it may take several hours for a coil to reach steady state surface temperatures, the majority of the temperature rise would be noticeable in the first hour.

Hazardous Locations and Explosive Atmospheres

Solenoid valves can be configured with certified explosion proof coils per UL-1203, CSA C22.2#30, and ATEX.  The copper coil wire may be encapsulated and isolated from any flammable gases that may be continuously present outside of the valve.  Flame proof designs consist of somewhat of a labyrinth so that even if flammable gas was ignited inside the coil, it would not propagate and ignite gas surrounding the valve.  For situations where flammable gases may be continuously present, electric motors are often not permitted due to sparking.  Taking this a step further, “intrinsically” safe coils with power outputs so low that they cannot ignite a gas may also be selected as well.  These, however, have very limited opening force due to such low power.  Industries concerned with flame propagation typically deal with natural gas processing and transportation, hydrogen refueling and fuel cells, methane recovery, and oil and gas in general.

Speed

Time to open and close is extremely fast on solenoid valves, and is often on the order of 50-300 milliseconds.  The smaller the valve, the quicker the small internal components can be made to move over short distances.  On piloted valves with larger traveling distances, viscous liquids tend the have the effect of slowing the movements down as internal components must push through those fluids.  Applications requiring a fast response time need look no further than solenoid valves.

Packless

Motor controlled valves and ball valves often have a stem that punctures the pressure boundary of the valve.  This stem is surrounded by a tight packing, often PTFE or graphite, to seal the high pressure fluid.  The packing may wear over time, and such wear may be exacerbated by shifting temperatures and ingress of contaminants.  Solenoid valves often have one motionless seam where the bonnet attaches to the valve body.  A gasket or o-ring of a material compatible with the working fluid can reliably be selected for this seam.  Radioactive applications sometimes do not allow the use of elastomeric gasket materials, so bonnets are welded to valve bodies.  Industries focused on minimizing fugitive emissions to reduce air pollution may opt for a packless solenoid valve.