SPECIAL-TASK VALVES PART 2 ... STEAM PRESSURE LETDOWN and CONTAMINANT BLOWDOWN VALVES

Blow the man down, bullies, blow the man downWey hey, blow the man down

("Blow The Man Down," Sea Shanty, first published in 1879)

MORE SPECIAL-TASK VALVES

Pressure Regulator on the upper left and Back Pressure Regulator on the upper right.

Brilliant PTOA Readers and Students ... meaning those who are reading the PTOA Segments in the intended, sequential order ... just learned about the special tasks that are performed by the Pressure Reducing / Regulating Valves and Back Pressure Regulating Valves which were featured in PTOA Segment #256.

This PTOA Segment features Special Task Valves called:

Steam Let-Down Valves.
Contaminant Blow-Down Valves and Blow-Off Valves.

STEAM SYSTEM LET-DOWN VALVES

The Steam Turbine (upper left) is the mechanical Driver, and the Compressor (upper right) is "The Load." The spinning action of the Turbine also rotates the blades of the Compressor. Other "Loads" that need to be rotated by a Driver include Electrical Generators and Centrifugal Pumps.

Every brilliant PTOA Reader and Student is already "knowed-up" about the Steam Turbines that were featured in PTOA Segments #195 through #201 as part of the PTOA Rotating Equipment Driver/Prime Mover Focus Study.

(Mechanical) Drivers are used to rotate (aka, spin) a "Load. " A "Load" might be an electrical generator or a dynamic pump or compressor.

And every PTOA Reader and Student also already knows that the Steam feed that enters a Steam Turbine is generated upstream in either a Package Boiler (featured in PTOA Segment #24 through #26) or a Waste Heat Boiler (featured in PTOA Segments #42 and #43).

This animated graphic shows how a Package Boiler heats Boiler Feed Water and converts the BFW into Steam that exits out above the Boiler.

The Package Boiler and the Waste Heat Boiler are designed to generate Steam at a high PV Pressure. For example, assume the Package Boiler in the nearby animated graphic is designed to generate steam at 150 psig.

The produced Steam will then be distributed via a High Pressure Steam Header to the end user(s) who demand high Pressure steam. One example of a high-Pressure Steam user would be the above-mentioned Steam Turbine that drives a Load.

Guess what?

Not all the steam users in an industrial complex require high PV Pressure steam!

Here are some industrial uses of steam that would be lower than 50 psig:

Steam Tracing on a segment of pipe that must be kept warm.

Steam Tracing that keeps Pipes and Valves from freezing.
Steam-out Steam needed to clean Vessels and Towers during a Turn Around prior to human entry.
Housekeeping steam used to clean the slabs that are used to support the processing plant's infrastructure.

Fred!

Would it be practical and efficient to have an entirely separate Boiler and Steam distribution system that delivers high PV Pressure steam to high-pressure steam users and low PV Pressure steam to low-pressure steam users?

The high PV Pressure of the Steam that flows through a header is reduced via venting some of the Pressure while flowing through a Steam Let-Down Valve.

HECK NO!

In the industrial processing environment ...

The job of the Let-Down Valve in a steam distribution system is to reduce the PV Pressure of the steam to a lower PV Pressure that can be safely used downstream.

The steam plumes from Let Down Valves are easy to see in this arial photo of a processing plant.

The high-pressure steam that exits the valve is jettisoned into the atmosphere.

The plumes of the "let down" steam are typically jettisoned high above the facility and out of harm's way regarding facility personnel.

The tell-tale sign of a Let Down Valve is the plume of steam vapor that appears to be jettisoned above a pipe rack as shown on left upper side of this photo.

Steam plumes become easier and easier to see as the ambient temperature gets colder and colder. These plumes are often erroneously cited in the media as air pollution. Albeit these plumes are hot and humid, they are composed of water vapor and contain no acid rain precursors or combustion gases.

CONTAMINANT BLOW-DOWN VALVES

Brilliant PTOA Readers and Students already learned about the contaminants (aka "heebie-jeebies") that build up in every circulated system, be the process fluid a liquid or a gas or a mixture.

The Blowdown on this graphic is shown in the bottom right quadrant. The amount of circulated water that is blown down is monitored with a Flow Meter.

The Cooling Water Utility system featured in PTOA Segments #39 and #40 focused on the build-up and removal of "heebie-jeebies" in the circulated water system.

Brilliant PTOA Readers and Students can thus accurately infer that contaminants will also build up in recycled gases, and circulated BFW in a Package Boiler, and circulated lubrication oil, etc.

The task of the Blow-Down Valve is to routinely remove circulated contaminates from recycled process streams.

The task of blowing contaminants out of a recycled system may be part of the Outside Operator's rounds or could be automized with an Intermittent Blow Down Valve.

In the olden days and in older processing facilities, Blow-Down Valves will take the form of quick-open Valves operated by a hand lever that the Outside Process Operator can quickly open and then close.

More modern facilities no longer rely on the Process Operator to remember to blow down process lines because the rotating equipment that is protected by clearing out contaminants is so expensive.

Blowing contaminants from a recycled process stream to a sewer can be performed automatically by actuating a Blow Down Valve. Actuating Valves for special tasks is featured in the next PTOA Segment.

PACKAGE BOILER BLOW-DOWN VALVES

In a Package Boiler, automated Blow-Down Valves are sometimes referred to as "Blowoff Valves."

Removing the suspended solids that circulate within the Package Boiler is crucial to maintaining Boiler performance.

This Process Operator safely and routinely opens a Blow Down Valve that removes circulated contaminants from a Boiler. The contaminants flow to the sewer.

Boiler Blowdown Valves will be located at a low point in the Boiler Water System piping.

The special tasks performed by Package Boiler Blowdown Valves are:

The suspended solids in BFW impact the BFW's Conductivity. This Boiler automatically blows down contaminants to sewer based upon the BFW Conductivity detected at the Water/Steam interface.

Removing suspended solids and impurities circulating in the BFW.
Rapidly lowering the BFW level in the Boiler when necessary.
Purging the acid solution that is used to clean Boilers out of the Boiler before it is returned to normal operations.

Furthermore, the performance and reliability of the Boiler greatly depends upon maintaining the desired PV Level of the Boiler Feed Water in the Steam Drum.

Package Boilers that are operated at pressures of 100 psi or more will either have two slow-opening Blowoff//Blow Down Valves or one slow-opening and one quick-opening Blowoff/Blow Down Valve.

This Boiler has two Fast Opening Blowdown Valves piped in parallel and one Slow Opening Blowdown Valve located downstream.

Under normal operating conditions, the slow-opening Blowoff / Blow Down Valve(s) will be used for discharging contaminants because using the slow-opening Blowoff / Blow Down Valve prevents shock to the Package Boiler system. The slow-opening Blowoff / Blow Down Valve is operated manually with a handwheel that requires 5 full turns to change from the fully closed to the fully open.

The quick-opening Blowoff / Blow Down Valve is reserved for emergencies which require rapid drainage.

TAKE HOME MESSAGES: The special task of a Steam Pressure Let-Down Valve is to reduce the PV Pressure of the high-Pressure Steam header to a lower PV Pressure that can be safely used by end users that need lower Pressure steam.

The special task of the Blow Down Valve is to remove contaminates that will materialize in any circulated fluid. Examples of circulated process streams include recycled gases, recycled lubricant oil, Cooling Water Utility water, etc.

Blow-Down Valves will be located in any circulating fluid system, typically placed where Rotating Equipment will be best protected. Blow-Down Valves may be manually operated by an Outside Operator doing his/her/their rounds. Modern facilities will have automated Blow Down Valves.

The Blow-Down Valve(s) for Package Boilers are sometimes referred to as Blowoff Valves. A Boiler will fail if the suspended solids and impurities are not routinely removed from the system.

Maintaining the BFW/Steam Level interface in the Boiler is also crucial to boiler performance. Boilers that produced at least 100 psig steam will have a blowdown system that includes slow-opening Blowdown / Blowoff Valves and a fast-opening Blowdown / Blowoff Valve. The slow-opening Blowdown / Blowoff Valve will require a full 5 turns to fully open, thus ensuring that the removal of circulated contaminants will be performed over a time interval that will not shock the system. The fast-opening Blowdown Valve is used for emergency draining.

Removing the impurities from a Package Boiler is crucial to the Boiler's performance and reliability. For this reason, modern Boilers will be equipped with automated Blow-Down / Blowoff Valves.

PTOA PV FLOWRATE FOCUS STUDY AREA
PIPING NETWORK HARDWARE

PIPING NETWORK HARDWARE

You need to login or register to bookmark/favorite this content.