Backflow Best Practices and Standard Details: Part 1

Sponsored by
Randy Holland
Premise Isolation Backflow Prevention:
Best Practices & Standard Details Part 1
Presented by

Part 3: The Differences Between DC & RPZ Backflow
Preventers
Premise Isolation Backflow Prevention:
Best Practices & Standard Details

Premise Isolation: Best Practices & Standard Details
….Definitions and term use:
Isolation backflow prevention: In addition to the lavatory and water fountain, most buildings’
plumbing systems include fixtures that are designed to clean contaminated equipment,
carbonate beverages, and even infuse chemicals and detergents. Many of these processes
create dangerous and toxic substances. If these substances were allowed to reverse back into
the building’s fresh water piping, an event known as backflow, it would create serious health
hazards for the individuals on site. Building authorities deal with these risks by specifying
appropriate backflow preventer assemblies at those specific locations where contamination is a
risk. The term for this solution is “isolation backflow prevention” because a special plumbing
apparatus known as a backflow preventer isolates high-hazard fixtures and equipment at the
point of use from the rest of the on-site piping system.

….Definitions and term use:
Containment or ‘Premise Isolation’ backflow prevention: For public water systems, water that
has been delivered through its water meter to a water customer is only done safely and
responsibly when there is no possibility that that water will return back from the customer to
the water system, an event (also) known as backflow. Disparate groups within plumbing,
design, and water management have devised their own favorite terms for this system. The
plumbing community prefers “Containment backflow prevention” because such systems
contain delivered water at the subscriber’s premises; On the other hand, water districts tend to
prefer “Premise Isolation”. It is important to understand that whether called Containment or
Premise Isolation, we are referring to the task of eliminating backflow at the Point of Supply
from the public water system.
This presentation is limited to the recognized best practices of these containment or premise
isolation systems.

Introduction
The water engineering community has been struggling with new
professional liability risk involving the location of premise isolation backflow
preventer systems; Not because of new design practices, but because of
new information about the old practices. There has been a slow trickle of
warnings for years, but in the past 3 years important organizations and
industry leaders have added new warnings with much stronger language
that not only change recognized best practices, but actually challenge the
fitness and safety of older placement methods altogether.

Introduction
Can we rid ourselves of the
problem by dumping the
system itself?
Sadly, we are learning
through SCADA and AMI
that there is actually more
backflow occurring at the
premise than we previously
suspected.

Introduction
With this new risk realization comes a new interested party: The insurance
company. Because of this very public commentary from experts and leading
groups, casualty carriers, through subrogation, have new weapons for
damage recovery. And anytime the accused designer is able to demonstrate
that local government contributed, whether materially or passively, to the
poor design, the water district and/or building authority may be at risk for
the liability.

Assuming the legal rights of a person for whom expenses or a debt has been paid.
Typically, an insurance company which pays its insured client for injuries and losses then
sues the party which the injured person contends caused the damages to him/her.
Introduction
Because of subrogation, the water district needs to
demonstrate that no unsafe methods are
promoted by their plans review teams. The best
way to demonstrate that is with published
standard details and drawings that are consistent
with recognized best practices.

Introduction
…Meanwhile, at the 2016-17 bi-annual conference of the American Society
of Plumbing Engineers, one popular learning module titled “Let the Civil
Engineer Deal with the Containment Backflow System” suggests that
leadership is seeking reassignment of the premise isolation backflow system
design to the civil discipline. No surprise, other than how long it took to
realize… Plumbing engineers have nothing to gain and everything to lose
when they specify indoor RPZs because
• The flood risks now being realized from indoor installations of RPZs is
extraordinary;
• Designing for outdoor placement includes grading and surface contouring
for sudden flood water flows; a task that is beyond the scope of a
plumbing engineer’s training or expertise.

Introduction
According to a survey of 1220 U.S. civil
and plumbing engineers conducted
over a 19‐month period, 3 out of 4 say
they need local water authorities to
provide standard details for outdoor
aboveground backflow preventer
systems.
You can read more about the results of
this survey here.

• Water Districts NEED
Premise Isolation in
order to fulfill their EPA
mandate; and
Introduction
Bottom Line:
“…. The return of any water to the
public water system after the water
has been used for any purpose on
the customer’s premises or within the
customer’s piping system is unacceptable
and opposed by AWWA.…”
• Premise-Isolation design details and specifications need to be provided to
civil engineers because of their general familiarity with standard details
and their comparable lack of familiarity with backflow systems.
AWWA’s preamble to the Cross Connection Control Manual,
published by EPA

1. Water utilities are seeking more
premise-isolation.
2. That more containment systems are
being specified as RPZ regardless of
hazard threshold.
3. AWWA, ASPE, & the legal community
recognize “outside aboveground” as
‘best practice’ for premise isolation.
This presentation will show…
Introduction

2 types of backflow Preventers:DesigndifferencesDCvs.RPZ
Double-Check Valve
Assemble, DC or
DCDA
Reduced Pressure Zone
Valve Assembly, RP
RPDA
A designer may specify one of two types of BFPs for premise isolation. Up
until recently, the decision for which assembly to specify was based solely on
the perceived hazard to the waste water system created by the processes of
the end user. High hazard (better named, high waste-hazard) uses were
required to utilize an RPZ. Uses that did not pose a risk to the waste water
were allowed to use a DC.
Design Differences DC vs. RPZ

2 types of backflow Preventers:DesigndifferencesDCvs.RPZ
Double-Check Valve
Assemble, DC or
DCDA
Valve Assembly, RP
RPDA
For example, a medical facility or a chemical plant triggered the requirement
for an RPZ while an office or simple retail user would be allowed to use a DC
or, depending on the municipality, no premise isolation system at all.
Now, as we will discuss below, many purveyors are requiring RPZs on all
premise isolation systems because of the inherent limits of protection provided
by the double check valve for the public water supply.

DC: Low hazard?
Public
(Supply)
side
Property
(Private)
side
Flow
DesigndifferencesDCvs.RPZ
The Double-check assembly was developed in the 1950s for the fire industry. And
for many years it was regarded as a satisfactory solution. The design is simple. Any
time system-water pressure on the property (private) side exceeds the system
pressure on the city (public) side, two redundant check valves close and water
stops flowing backwards.

DC: Low hazard?DesigndifferencesDCvs.RPZ
But no remedy exists in the event of a malfunction of the valve closures or if
debris in the water line causes the valves to not close completely. Additionally, the
DC is a closed, or blind system making detection of any failure impossible without
a field test performed by a licensed tester. Today, millions of DCs are in service that
may have failed. When a Florida city began its annual testing program in 2010, it
found 52% of the valves in service had failed with no way to determine how long
they had been inoperable.
Public
(Supply)
side
Property
(Private)
side
Flow

RPZ: Fail-safe against returning water
Flow
Property
(Private)
side
Public
(Supply)
side
The RPZ emerged in the 1970s as a remedy to the double-check limitations. Like
the DC, it incorporates 2 redundant check valves. But unlike the DC, the RPZ
incorporates a hydraulically operated differential relief valve directly beneath the #
1 check valve. It is this relief valve’s placement (along with the universal laws of
hydraulics) that make this a fail-safe solution for water purveyors. As elegant as
the design is, it comes at a cost. And that cost is the surrounding area.

Flow
Property
(Private)
side
Public
(Supply)
side
As the DC reveals, valves fail. But when they fail in an RPZ, the assembly is
designed to create a deluge event directly under the assembly so that no
contaminated water returns to the public water supply. Because of the danger of
contamination, no water from the relief valve may be piped directly from the
assembly. It must release into the atmosphere away from any piping. Watch this
short video revealing an actual discharge.

Flow Stop
In a flow-stop situation the water
between the check valves will often
drain out the relief valve. Some
think that that event defines the
limit of what water can ever flow
into a drain.
Not so.

Loss of pressure
#2
valve
blocked
Consider a flow-stop situation, one that
might naturally occur at the end of the
day. If you look closely, you can see that
a small pebble has lodged in the #2
check valve. Now let’s say there’s a fire
around the corner that causes back
siphon at this point in the system.
Because the # 2 check valve is not
closing, all the water that has been
delivered to the building will continue to
flow out the relief valve until the private
lines are cleared. If this is a four story
building, that’s a lot of water!

#1
valve
Failure
Normal
delivery
pressure
Now consider a failure of the #1 check
valve. Under normal operating
conditions, this failure would go
unnoticed. After all, water is being
called for by the user through the
opening of taps. The water flows in
undeterred.
But with this imbalance in the system,
changes in demand tend to rock the
remaining valves open and closed
sporadically.
Demand

#1
valve
Failure
Blockag
e relief
valve
Demand
Normal
delivery
pressure
This creates the conditions for the
“perfect storm” scenario. The imbalance
created by the # 1 failure makes the
relief valve more prone to opening
momentarily, allowing debris to block
the closure of that valve.
Under such conditions, a constant flow
of delivered water will begin to flow
directly out the relief valve. This
reduces water pressure for the user, but
delivery will continue.

No
demand
Normal
delivery
pressure
The real damage begins when the user
stops using water such as at the end of
a work day.
With the relief valve blocked open and
the # 1 valve inoperative, all the water
that the purveyor can provide will flow
unabated out the relief valve wherever
it might be, and continue until the water
source is interrupted.
This is the scenario that must be
avoided: the perfect storm.

Double-Check Valve
Assemble, DC or
DCDA
Valve Assembly, RP
RPDA
Safe-T-Cover has put together an easy to reference guide on the differences
between double checks and reduced pressure zone backflow preventers on
their blog.

 The public water supply is unprotected from returning water without a
premise isolation system. RPZs are only fail-safe solution.
 The duties of the building/plumbing authority and the plumbing code do not
wholly satisfy the duties of the water utility.
 Indoor RPZs 3” and larger are perpetual floods risks.
 The need to address sudden on-site water flows disqualify MEPs from outdoor
premise isolation design, even if within MEP halo.
 Civil engineers are unfamiliar with BPA installations and need standard
details from water authorities.
Take-Aways
 A broadly adopted region-wide set of guidelines would save cities 100s of
hours in plans-review time.

 Safe-T-Cover's blog: Updated weekly with articles on backflow prevention,
standard details, and best practices.
 Enclosure Design eBook: Learn the 5 design considerations for
aboveground enclosures
 Recent story on decision to add standard details by the city of Arlington, Texas
 Trends in Backflow Preventer Installation: A downloadable guide to the latest
trends in backflow best practices.
Additional Resources

Thank You!

Backflow Best Practices and Standard Details: Part 1

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