# 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 end

devices 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

A control loop has the following components:

• Pneumatic supply gas

• Pneumatic controller

• Process variable /
measurement

• Valve actuator

• Control valve

# Simple P rocess 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

# 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 C ontrollers 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 & l ength 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

15 

###  

## 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 loo p

# 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 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

17  

### 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 de

actuation

• 

### 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

21

Note: data is not actual; constructed for illustration purposes only Zero output emission rate (steady

- state) determined by restrictive orifice size, gas pressure, and gas properties

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 low

bleed 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

# 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 pilot

plug 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 end

device 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

31 Schematic Courtesy of FISHER

# 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 )