Pneumatic Controllers
Common Misconceptions 

2

 Intermittent controllers emit at a higher rate than low 

bleed (Subpart W)

 Low bleed controllers are the best overall choice

 Emissions also occur at the valve actuator

 Controller terminology is standardized and commonly 

understood

 That snap-acting, on/off, no-bleed, & intermittent are 

synonymous terms

 That throttling, continuous bleed, and proportional are 

synonymous terms
Goals 

 Seek common understanding of pneumatic controllers

 Types of pneumatic controllers

 How they operate

 Expected emission profiles. 

 Seek common understanding of key principles

 The choice of controller types for a particular application is dictated 

by the process needs/demands while ensuring safe and reliable 

operation

 For most applications, intermittent controllers are the lower 

emission option than continuous bleed controllers

 If the process can tolerate the inherent delay in response associated 

with a low-bleed continuous controller, a low-bleed continuous 

controller may, in limited situations, be a lower emission option

3
Process Control Systems

4

 A process control system/loop must have:

 A way to sense the state of a process variable (Float, pressure transducer, etc.)

 A way to change the state of the sensed process variable (Pneumatic Supply Gas, 

Controller, Actuator, and Valve)

 Process variables most commonly controlled in upstream Oil & Gas are:

 Fluid level (often found on separators, tanks, treaters, etc.)

 Pressure (includes pressure regulating, back pressure regulating, and overpressure limiting)

 Temperature (includes tank heaters, indirect process heaters, direct process 

heaters, and fan control)

 Differential pressure (often used as a surrogate for flow, generally used for 

constant flow processes)

 Position (includes devices that sense plunger arrival in a well and signal enddevices to allow after flow and/or to shut off the flow to allow the plunger to 

drop)

 Safety – Unique Category (includes control of emergency shut-down valves 

that shut when manually tripped or an unsafe condition is sensed)
Simple Control Loop Example - Wrong 

5
Simple Control Loop Example - Right

6

High Pressure

Pneumatic Supply Gas

(15 -35 psig typical) Bleed (Continuous)

or

Vent (Intermittent)

A control loop has the 

following components: 

• Pneumatic supply gas

• Pneumatic controller

• Process variable / 

measurement

• Valve actuator

• Control valve
Simple Process Control Loop Concepts

7

 The sensor, control valve and actuator do not bleed or 

vent gas

 All emissions occur at the controller

 The amount of gas needed for a full actuation cycle 

depends on the pressure and volume of the control loop

 The full actuation volume is the volume of the valve actuator + 

the tubing from the controller to the actuator – corrected to 

standard conditions for the actuation pressure. 

 For a continuous bleed controller the speed of actuation 

is limited by the rate that gas can pass through the 

restrictive orifice. (Limits utility of low bleed devices)
Example: 3-Phase Separator Control Schematic

8

Figure 1 End Device1
Pneumatic Controllers

 Pneumatic controllers convert an input signal to a pneumatic 

pressure output using gas pressure

 Input signals: Mechanical (e.g. float arm (level)), (bordon tube 

(pressure)), (bi-metal rod (temperature), etc)); Electrical (e.g. pressure 

transducer)

 Two types of pneumatic controllers:

 Intermittent vent (physical barrier between supply gas and output)

 Continuous bleed (no physical barrier)

 Two types of service

 On/Off

 Throttling

 Protection/Shut-in Devices – unique category

9
Controller Description

10

 Large number of controller manufacturers and models

 Large number of field configurations

 All typically control a valve via a valve actuator

 From an emissions standpoint, process controllers can be 

classified into 4 buckets

Type of Service

On/Off Throttling

Type of 

Controller

Intermittent Vents on de-actuation with 

emissions near zero between 

de-actuation cycles

Vents some gas pressure 

when valve needs to 

move towards closed

Continuous Bleeds continuously, rate slows 

while process is “on”, but 

average rate is ~constant

Bleeds continuously, rate 

varies with actuation, but 

average rate is ~constant
On/Off Controller Action – Snap acting vs. Proportional

11

 On/Off Controllers can be configured to operate in a “snap-acting” or proportional mode. 

These modes are often confused with the on/off and throttling services but are not 

interchangeable with them.

 Snap-acting is not the same as On/off service

 Proportional acting is not the same as Throttling service 

 On/Off Snap acting (like an on/off light switch)

 Snap acting controllers wait until a set point is reached and then send a full supply pressure signal to the 

valve actuator which fully opens the valve.

 When the low set point is reached, a snap acting controller will fully vent the actuation volume and return 

the valve to fully closed.

 On/Off Proportional (like a light switch with a dimmer)

 A proportional controller will send a partial pressure signal to a valve actuator as the control variable 

reaches the actuation set point. If the control variable continues to increase, the controller will increase 

the pressure signal to the valve actuator until the variable ceases increasing. 

 When a control variable approaches the de-actuation set point a proportional controller will vent a 

portion of the pressure signal from the actuator. As the control variable continues to decrease the 

controller will decrease the vent rate until the actuator loop is fully depressured. 

 This is different than a throttling action and can be thought of as a soft open/close action

 Snap acting vs Proportional action does not normally have a large impact on emissions. 

 For an intermittent-vent controller, a proportional action may vent somewhat less gas on de-actuation than 

a snap-acting controller in the same service.

 For a continuous bleed controller emissions would be essentially the same between proportional and 

snap-acting settings
Intermittent Controllers – How they operate

12

 On/Off service

 Operate against a set point (s) (e.g. liquid level Hi/Lo)

 Send a pressure signal (gas) when a set point (e.g. liquid level) is 

reached (e.g. pressure-to-open dump valve) to fully open valve

 Vent pressure (gas) to fully return valve to prior position (e.g. 

normally closed with spring-for-return dump valve)

 Throttling service (like cruise control in your car)

 Operate against a desired set point (e.g. pressure)

 Send a pressure signal (gas) to increase output signal (pressure to 

open – partially open valve) 

 Vent pressure (gas) to decrease output signal (partially close valve)

 Hold a valve in an intermediate position
Intermittent Controllers – Design & Operation

13

 Typically a pilot (small 3-way valve) operated design

Pilot Ball

Upper Seat

Output Port

Thrust Pin

Supply Port

• To actuate, the thrust pin displaces the upper ball allowing 

supply gas to flow to the output port and valve actuator

• To de-actuate, the thrust pin moves down, the upper ball 

reseats and the exhaust port is opened de-pressuring the 

actuator and tubing

• In throttling service the diaphragm senses force/feedback with 

the output signal proportional to the mechanical force on the 

thrust pin. The upper ball controls supply gas and has spring 

assist to help seating and the lower ball vents gas. The pilot 

seeks to maintain equilibrium by increasing or decreasing 

pressure. Supply air does not flow when balanced.

Exhaust Port Not Shown

Images courtesy

of Norriseal

Diaphragm

Lower Ball

Assist Spring
Intermittent Controllers – Emissions #1

14

 Two components to emissions in normal operation

 Vent on de-actuation – Almost all emissions from an intermittent 

controller are from venting the internal volume from the valve actuator 

to return the valve to its prior position and return the control loop to 

an idle (non-emitting) mode. 

 Seepage/Leakage past pilot ball/seat – Normally minor

 De-actuation vent volume depends on:

 Size/volume of valve actuator & length of valve actuator travel –

Actuation Volume

 Service

• On/Off - Pressure of output gas (function of supply pressure and controller); 

• Throttling – Differential pressure from adjustments to maintain set-point 

 Length and diameter of tubing from controller to actuator
Intermittent Controllers – Emissions #2

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 Emissions from intermittent controllers are “event based”

 An event is an occurrence of cycling the control valve actuator

 Nearly zero emissions between events

 Gas volume emitted per unit of time depends on frequency of 

cycles and de-actuation volume per cycle

 Total vented gas is a function of the number of times that the 

valve operates, not the length of time the controller is in 

operation

 Normal continuous seepage/leakage depends on:

 Material of balls and seats; Elastomer seats claim zero leakage

 Pressure of supply gas

 Gas gravity; Natural gas volume is ~1.3 X air volume 

 Designed for near-zero emissions when not de-actuating a control loop
Emission Profile

16

On/Off Snap Acting

Throttling

More frequent de-actuations 

with less volume each

Note: data is not actual; constructed for illustration purposes only

0.000

0.050

0.100

0.150

0.200

0.250

0.300 14:44:00 14:44:20 14:44:40 14:45:00 14:45:20 14:45:40 14:46:00 14:46:20 14:46:40 14:47:00 14:47:20 14:47:40 14:48:00 14:48:20 14:48:40 14:49:00 14:49:20 14:49:40 14:50:00 14:50:20 14:50:40 14:51:00 14:51:20 14:51:40 14:52:00 14:52:20 14:52:40 14:53:00 14:53:20 14:53:40 14:54:00 Gas Emitted (SCF/Min)

Minutes and Seconds

Intermittent Controller - On/Off Service

De-actuation Vents

Near-zero leakage

0.000

0.050

0.100

0.150

0.200

0.250

0.300

14:44:0014:44:2014:44:4014:45:0014:45:2014:45:4014:46:0014:46:2014:46:4014:47:0014:47:2014:47:4014:48:0014:48:2014:48:4014:49:0014:49:2014:49:4014:50:0014:50:2014:50:4014:51:0014:51:2014:51:4014:52:0014:52:2014:52:4014:53:0014:53:2014:53:4014:54:00

Gas Emitted (SCF/Minute)

Intermittent - Throttling

De-actuation Vents

Near-zero leakage

Seep/Leak Rate 

examples

Kimray: (Air)

Snap: 0.407 scf/day

Throttle: 0.610 scf/day

Norriseal: (Gas)

Snap: 0.2 scf/hr

Throttle: 0.007 scf/hr

Envirosave: 0.0 scf/hr
Cycle Frequency Challenges

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 Events do not tend to occur on a predictable schedule

 A pressure regulator is actuated when downstream demand changes (not 

predictable)

 A level controller is actuated when liquids accumulate (a function of the 

process inlet stream which is not completely predictable for a short time 

frame)

 Two identical controllers and end-devices can have significantly 

different emissions depending on the particular process 

configuration, process variable controlled and demands that change 

the number of cycles

 There is no reliable way to convert an event-based emission into a 

time-based emission rate unless the frequency of events (actuations) 

is known or determined. 

 Currently, there is insufficient high-quality measurement data, 

coupled with the relevant process data, to support an average 

emission factor reflective of the population of intermittent 

controller types, services, and conditions. 
Continuous Bleed Controllers 

Design & Operation

18

• As the block/flapper is pushed closer 

to the nozzle the output signal to the 

actuator increases

• As the block/flapper moves away 

from the nozzle the output signal 

decreases

• Maximum emissions are controlled by 

the restriction orifice

Typically a nozzle & flapper arrangement
Continuous Bleed Controllers – How they operate

19

 On/Off service

 Operate against a single set point (e.g. pressure)

 Fully close the nozzle (with flapper) and send a 100% output pressure signal (gas) 

when a set point (e.g. pressure) is reached 

 Continuously bleed gas at a rate determined by critical orifice/restriction size; 

gas pressure; gas gravity and degree of nozzle closure by block/flapper - except 

when nozzle is fully closed 

 Throttling service

 Operate against a desired set point (e.g. pressure)

 Partially close nozzle and send a pressure signal (gas) to increase output signal 

(partially open valve)

 Partially open nozzle to decrease output signal (partially close valve)

 Continuously bleed gas at a rate determined by critical orifice/restriction size; 

gas pressure; gas gravity and degree of nozzle restriction 

 Continuous “low bleed” <6scf/hr; “high bleed” >6scf/hr (regulatory)

 Low bleed rates are achieved by using small diameter orifices (0.012 – 0.009 

inches). These small diameters increase the risk of debris plugging and 

significantly limit the speed of control action/response which limits the situations 

they can be used. 
Continuous Bleed Controllers - Emissions

20

 Emissions in normal operation

 Bleeds to atmosphere continuously except for 100% output (flapper 

fully closes nozzle)

 Rate of bleed decreases during actuation and increases on deactuation

• De-actuation exhausts the actuation volume plus returns controller to 

normal bleed rate

 Bleed rate depends on

 Restrictive orifice size

 Pressure of supply gas

 Gravity of supply gas 

 Lower bleed during actuation depends on degree that flapper closes 

nozzle vs. maximum rate through restrictive orifice

 Exhaust during de-actuation balances lower rate during actuation
Continuous Bleed Controllers

Emission Profile

Note: data is not actual; constructed for illustration purposes only 21

Zero output emission rate 

(steady-state) determined by 

restrictive orifice size, gas 

pressure, and gas properties

0

0.05

0.1

0.15

0.2

0.25

0.3

14:44:0014:44:2014:44:4014:45:0014:45:2014:45:4014:46:0014:46:2014:46:4014:47:0014:47:2014:47:4014:48:0014:48:2014:48:4014:49:0014:49:2014:49:4014:50:0014:50:2014:50:4014:51:0014:51:2014:51:4014:52:0014:52:2014:52:4014:53:0014:53:2014:53:4014:54:00

Gas Emitted (SCF/Minute)

Continuous Low Bleed

Actuation Cycles

De-actuation Exhaust

0

0.05

0.1

0.15

0.2

0.25

0.3

14:44:0014:44:2014:44:4014:45:0014:45:2014:45:4014:46:0014:46:2014:46:4014:47:0014:47:2014:47:4014:48:0014:48:2014:48:4014:49:0014:49:2014:49:4014:50:0014:50:2014:50:4014:51:0014:51:2014:51:4014:52:0014:52:2014:52:4014:53:0014:53:2014:53:4014:54:00

Gas Emitted (SCF/Minute)

Continuous - High Bleed

Actuation Cycles

De-actuation Exhaust

The average emission rate over 

time will be ~equal to the 

amount of gas through the 

restrictive orifice 
Protection/Shut-in Devices/Controllers

22

 Not a source of material emissions

 Used to protect against abnormal 

conditions

 e.g. Overpressure/under-pressure 

(Hi/Lo pilots); Hi/Hi liquid level in 

tanks/vessels; 

 Typically do not emit unless 

abnormal condition occurs

 Example: Hi/Lo pressure pilot

 Normally operates at 100% output 

holding a “fail closed” valve open

 If overpressure or under-pressure 

occurs then output bleeds to 

atmosphere and valve closes

 Not controlling a “process variable”

 Seldom open and emit

Image and information courtesy of FISHER
Intermittent or Continuous – Choice?

23

 Although intermittent vent controllers are the lowest emitting 

selection in most applications, due to not bleeding gas between 

actuation cycles, in certain cases a low bleed continuous type 

controller may be a lower emission selection.

 This is only the case where “over-shooting” the high parameter set point 

can be safely and operationally tolerated.

 Each application must be analysed to determine whether a lowbleed continuous controller is a better emission choice and 

whether the application can tolerate the “overshoot” inherent in 

the slow response time of a low bleed continuous controller. 

 All of the different types and services of controllers have 

scenarios where they are the lowest emission options and 

operators need the flexibility to deploy the appropriate 

technology while taking into account the overriding drivers of 

operational and process safety, stability, and reliability. 
Intermittent or Continuous – Temp. Control Example

24

 Comparing a typical temperature control loop using 

an intermittent controller to a low-bleed continuous 

controller the intermittent is the lower emission 

choice

 Temperature control loops sense temperature and 

control fuel flow to a heater to turn it on or off. 

Very common and basically every field heater 

(dehydrator, tank heater, heater-treater, production 

unit, etc.) has a temperature control loop. Also used 

to control air coolers and other processes 

 Since the fuel valve is small and in low pressure 

service, the actuator used to move the valve is 

equally small volume. For instance an actuator for a 

fuel control valve is about 1.1 cubic inch volume 

which will yield emissions ~100 times smaller per 

cycle than the 110 cubic inch level controller 

example used in background slide.

• It would require ~378 cycles/hr (or about one cycle every 9.5 seconds) to reach 

hourly emissions of 6 scf/hr (regulatory low-bleed continuous). This is not going to 

occur. 

• In this application, an intermittent type controller is the obvious low emission choice 

and fits the process control demand very well. 

Image Courtesy of Kimray
Emission Factors and Population Counts

25

 Subpart W (GHGRP) emission data for pneumatic controllers

 The current emission factor (EF) for intermittent controllers is 

almost certainly an order of magnitude too high. 

 Given how intermittent controllers and continuous controllers operate this is 

almost certainly not valid across the spectrum of controllers and services.

 For 2011 and 2012 the “counts” of pneumatic controllers in each category were 

estimated rather than counted. The 2013 emission data requires an actual count. 

 National inventory emission data for pneumatic controllers

 The National Inventory uses a single estimate of the number of pneumatic 

devices and a single emission factor to calculate a potential methane emission for 

pneumatic controllers (called devices).

 Reductions in emissions attributable to Natural Gas Star voluntary taken and 

reported actions and regulatory required reductions (if required) are subtracted 

from these potential emission estimates.

 It is not clear what the genesis is for the pneumatic controller population 

estimate or emission factor. 
Known Studies/Projects

26

 UT Phase 1 Study sampled 305 controllers.

 UT Phase 2 study, with improved methodologies, is underway and due to be 

published later this year

 GRI/EPA 1996 study

 Canadian Association of Petroleum Producers published pneumatic device 

emission factors (CAPP, 2002 and CAPP, 2003)

 Recent British Columbia study (Prasino) published

 Pioneer Natural Resources is planning to publish the results of a measurement 

study in the near future. 

Important to remember the following:

 All studies need to be QA/QC’d to judge whether the type of controller and 

service was correctly identified 

 All studies need to be QA/QC’d to judge whether the appropriate 

methodologies, sampling populations, and sampling times were used to 

adequately measure emissions from controllers

 All studies need to be QA/QC’d to judge whether the data collected was 

appropriately analyzed and attributed
Conclusions

27

 Operators need the flexibility to deploy the appropriate technology while taking 

into account the overriding drivers of operational and process safety, stability, and 

reliability. 

 Different types and services of controllers have scenarios where they are the 

lowest emission options 

 Intermittent controllers will be the low emission choice in most applications

 In a small number of cases, where the process can tolerate control 

delay, low-bleed continuous controllers may be the low emission choice.

 The current views regarding the emission rate from intermittent controllers 

are almost certainly not correct.

 Although measurement studies are being done and published, care needs to 

be taken to QA/QC the results before simply accepting them. Sufficient good 

quality study and measurement data is not yet available to characterize the 

population of controllers and applications. 

 Due to the methodologies and emission factors the GHGRP and National 

Inventory data for pneumatic controllers needs to be carefully understood 

before using it for either population counts of different types of controllers or 

emissions estimates. 
Background

28
Intermittent Controllers – Emissions Example 

29

 Typical example only. In practice the 

parameters and actuations per unit of 

time will vary. 

 The seepage/leakage of gas (minor) 

while in the idle mode is additional to 

the volume shown

 In 2012 the average US gas well 

produced 1.3 bbls/day of 

oil/condensate (Source: DI)

 In 2012 the average US oil well 

produced 9.4 bbls/day of oil (Source: 

DI)

 In 2012 the average US well produced 

5.6 bbls/day of oil (Source: Basic 

Petroleum Databook)

 At 25 psig and 5.6 bbls/day the gas 

emitted for an average intermittent oil 

dump controller would be between 

1.4 SCF/hr and 0.4 SCF/hr; The 

GHGRP factor is 13.5 SCF/hr

2" Level 

Range

6" Level 

Range

Supply Pressure PSIG 25 25

Atmospheric Pressure PSI 14.7 14.7

Gallons per dump 1.3 3.9

Production/day (bbls) 10 10

Length of tubing (ft) 10 10

Dia. Of tubing (inches) 3/8 3/8

X-section of tubing (sq ft) 0.0008 0.0008

Volume of tubing (scf) 0.022 0.022

Volume of actuator (cubic inches) 110 110

Actuator de-actuation volume (scf) 0.17 0.17

Total volume per cycle (scf) 0.19 0.19

# of Cycles per hour 13.5 4.5

Total volume per hour (scf) 2.6 0.9

On/Off Level Controller/Device Example
Intermittent Controllers – Emission Variability

30

 What can go wrong to increase emissions

 Debris/deposits on vent pilot plug: Material on the vent pilot can 

allow the controller to exhaust gas during the activation cycle. 

 Debris/deposits on the supply pilot plug. Material on the supply pilot 

can cause the introduction of gas while the vent is open.

 Broken spring (when equipped) The spring holds the supply pilotplug on its seat and without this spring the controller has similar 

emissions as a continuous bleed controller. This particular 

malfunction generally calls attention to itself quickly because the enddevice being actuated doesn’t operate.

 Broken diaphragm (where installed). Many intermittent vent 

controllers have diaphragms for various reasons. A detailed analysis 

of a particular device would be required to determine the results of 

the failure on the emitted gas.

 Other - leaking tubing/tubing-fittings, etc (equipment leaks not 

controller emissions) 
Continuous Bleed Controller - Schematic

Schematic Courtesy of FISHER 31
Continuous Bleed Controllers

Emission Variability

32

 What operational factors impact emissions rate

 Supply Gas Pressure and Gravity

 What can go wrong to change emissions

 The orifice can partially plug which lowers bleed rate

 A continuous controller that will still operate an end device 

has its emissions capped by the restriction orifice

 A leak within the controller upstream of the restriction orifice 

can increase total emissions.

 Other (e.g. leaking tubing/tubing-fittings, o-rings)