POWER November 2013 - 18

New Design Solves
Scaling Problems on
Geothermal Control Valves
Scaling is one of the most frequently occurring
problems in geothermal power
plants and can prohibit the control of well
flow if it builds in the well or wellhead.
At HS Energy on the Reykjanes Peninsula
in Iceland, the problem was minimized
utilizing a design developed by a veteran
plant worker. The solution could not
have been implemented without an openminded
leader who was ready to listen and
push forward with the remedy. In this particular
case, Erlendur Guðmundsson's idea
required the backing of HS Energy Vice
President Albert L. Albertsson to get the
ball rolling.
The solution that was developed is a new
valve, referred to as Elli-valve, which allows
for better flow control and easier cleaning.
It also has been shown to have economical
advantages over other valves on the market.
This solution could be useful in other
geothermal power plants dealing with similar
issues, but it also demonstrates how
the ingenuity of the people most familiar
with an issue, such as the plant workers,
can be used to provide effective solutions
to problems in any plant.
The Reykjanes Geothermal
Field
On the Icelandic Reykjanes Peninsula,
HS Energy operates two geothermal
power plants, Svartsengi and Reykjanes.
Svartsengi currently produces 46.4 MWe
and 150 MWt. The high-temperature area
stretches over 2 square kilometers. The
Svartsengi geothermal power plant was
constructed in small phases beginning in
1974 and stretching out to 2000. Reykjanes
geothermal power plant went into
production in 2006. It currently operates
two 50-MW dual-flow turbines delivering
approximately 100 MWe. Both plants are
relatively close to the ocean, which results
in brine rich in Na, Cl, K, SiO2
and SO4
. A
detailed description of the brine can be
found in Table 1.
At Svartsengi, the average flow rate
is approximately 226 kg/s (as reported
in a 2000 study) where steam is 32% of
the flow at 5.5 bar separation pressure,
and temperatures have been shown to be
around 240C. At Reykjanes, temperatures
typically run between 250C and 320C.
Studies have shown that the chemical
18
composition of the brine increases certain
problems in the control valves. (Interestingly,
the scaling problem experienced in
the control valves at HS Energy has not
been seen at such magnitude at other
geothermal power plants located relatively
nearby in Iceland, namely Hellisheidi and
Nesjavellir-likely because the chemical
composition of the brine is very different
at those other locations.) Previous studies
conducted on surface pipes in the Reykjanes
area, operating at pressures of 20
and 5 bar-g, have recorded scale forming
at a rate of 0.1 to 0.5 mm in 30 days or
roughly 1.2 mm to 6 mm per year. Scaling
eventually leads to difficulties in controlling
the flow from the well.
Types of Valves
A geothermal wellhead assembly consists
of many different valves with variTable
1. An example of the brine
chemical composition at Reykjanes
and Svartsengi. Source: HS Energy
mg/kg
pH/C°
SiO2
Na
Cl
K
SO4
Al
F
B
Ca
As
Ba
Fe
Zn
Cu
Mg
Pb
Ni
Cd
Cr
Hg
Sr
Mn
H2S
CO2
5.9/275
601
9,470
18,900
1,410
18
0.0809
<0.2
7.7
1,600
0.0824
8.93
0.0447
0.0196
0.000635
0.486
<0.00414
<0.000587
<0.000011
0.000156
<0.000002
9.4
2.01
23
1,060
www.powermag.com
Reykjanes Svartsengi
4.91/238
440
6,487
13,011
953
26.7
0.138
0.192
6.6
1,054
-
-
0.051
-
-
0.321
-
-
-
-
-
-
-
10.5
470
POWER | November 2013
ous functions. Most of the valves are
not operated regularly. Many valves are
used only when necessary to divert the
flow from the collection pipe system,
close the hole if it is not used in production,
or when isolation is necessary
for repairs.
The control valve is regularly operated,
however, where its purpose is to
control the flow of steam and brine from
the well. Controlling the flow from the
well is essential for efficient operation
of a geothermal power plant. Choosing
the right control valve is therefore of
great importance.
Issues with Geothermal
Control Valves
The problem of scaling is hard to avoid.
Because it can hinder efficient operation
of the power plant, it warrants special attention.
During the early operational years
at HS Energy, a butterfly valve, which had
been modified to compensate for the
harsh geothermal environment, was used
to control the well flow manually. A butterfly
valve is not well suited to throttling
and is simply a disk that can be rotated on
an axis to control flow (Figure 1).
The bearing house of the butterfly
valve was normally cooler than the brine
that flowed around it, which led to scaling
on the bearing house and the bearings.
Once the scale formed, control of
1. A fully opened butterfly valve.
Courtesy: Reynir S. Atlason
http://www.powermag.com
POWER November 2013

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